Determining the missing digits of $15! equiv 1square0767436square000$ without actually calculating the...












6












$begingroup$



$$15! equiv 1cdot 2cdot 3cdot,cdots,cdot 15 equiv 1square0767436square000$$



Using a calculator, I know that the answer is $3$ and $8$, but I know that the answer can be calculated by hand.




How to calculate the missing digits? I know that large factorials can be estimated using Stirling's approximation:
$$15! approx sqrt{2picdot 15}
cdot left(frac{15}{e}right)^{15}$$

which is not feasible to calculate by hand.



The resulting number must be divisible by 9 which means that the digit sum must add up to 9 and is also divisible by 11 which means that the alternating digit sum must be divisible by 11:



$1+ d_0 + 0 + 7 +6 +7 +4 +3+6+d_1+0+0+0 mod phantom{1}9 equiv ,34 + d_0 + d_1 mod phantom{1}9 equiv 0 $
$-1+ d_0 - 0 + 7 -6 +7 -4 +3-6+d_1-0+0-0 mod 11 equiv d_0 + d_1 mod 11 equiv 0 $



The digits $3$ and $8$, or $7$ and $4$, fulfill both of the requirements.










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$endgroup$








  • 8




    $begingroup$
    Well, you know that the sum of the digits is divisible by $9$ and the alternating sum of the digits is divisible by $11$. That should tell you what the two digits are, though not their order. But you should be able to argue that the first non-zero digit from the right must be even.
    $endgroup$
    – lulu
    Jan 2 at 21:36










  • $begingroup$
    Why is the sum of the digits divisible by 9?
    $endgroup$
    – Darkice
    Jan 2 at 21:37






  • 3




    $begingroup$
    @Darkice because for every natural number $ngeq 6$ you have that $n!$ is a multiple of nine and a natural number is a multiple of nine if and only if the sum of its digits is a multiple of nine.
    $endgroup$
    – JMoravitz
    Jan 2 at 21:41








  • 1




    $begingroup$
    Alright, now I know that the digits must add up to a multiple of 9 and must be divisible by 11. So the digits could be 7 and 4 or 3 and 8 which would both fulfil the requirements.
    $endgroup$
    – Darkice
    Jan 2 at 21:59






  • 6




    $begingroup$
    For your specific case, one of the numbers you are looking to find is the final nonzero digit. As $15! = 2^{11}cdot3^6cdot5^3cdot7^2cdot11cdot13$ the final nonzero digit will be $2^8cdot3^6cdot7^2cdot11cdot13pmod{10}$ which is a much easier calculation to do modulo 10 than fully expanding out the product. $2^8cdot3^6cdot7^2cdot11cdot13equiv 6cdot 9cdot 9cdot 1cdot 3equiv 8pmod{10}$. This would be admittedly more difficult if both of the numbers were located further in the middle and not next to the trailing zeroes.
    $endgroup$
    – JMoravitz
    Jan 2 at 22:09


















6












$begingroup$



$$15! equiv 1cdot 2cdot 3cdot,cdots,cdot 15 equiv 1square0767436square000$$



Using a calculator, I know that the answer is $3$ and $8$, but I know that the answer can be calculated by hand.




How to calculate the missing digits? I know that large factorials can be estimated using Stirling's approximation:
$$15! approx sqrt{2picdot 15}
cdot left(frac{15}{e}right)^{15}$$

which is not feasible to calculate by hand.



The resulting number must be divisible by 9 which means that the digit sum must add up to 9 and is also divisible by 11 which means that the alternating digit sum must be divisible by 11:



$1+ d_0 + 0 + 7 +6 +7 +4 +3+6+d_1+0+0+0 mod phantom{1}9 equiv ,34 + d_0 + d_1 mod phantom{1}9 equiv 0 $
$-1+ d_0 - 0 + 7 -6 +7 -4 +3-6+d_1-0+0-0 mod 11 equiv d_0 + d_1 mod 11 equiv 0 $



The digits $3$ and $8$, or $7$ and $4$, fulfill both of the requirements.










share|cite|improve this question











$endgroup$








  • 8




    $begingroup$
    Well, you know that the sum of the digits is divisible by $9$ and the alternating sum of the digits is divisible by $11$. That should tell you what the two digits are, though not their order. But you should be able to argue that the first non-zero digit from the right must be even.
    $endgroup$
    – lulu
    Jan 2 at 21:36










  • $begingroup$
    Why is the sum of the digits divisible by 9?
    $endgroup$
    – Darkice
    Jan 2 at 21:37






  • 3




    $begingroup$
    @Darkice because for every natural number $ngeq 6$ you have that $n!$ is a multiple of nine and a natural number is a multiple of nine if and only if the sum of its digits is a multiple of nine.
    $endgroup$
    – JMoravitz
    Jan 2 at 21:41








  • 1




    $begingroup$
    Alright, now I know that the digits must add up to a multiple of 9 and must be divisible by 11. So the digits could be 7 and 4 or 3 and 8 which would both fulfil the requirements.
    $endgroup$
    – Darkice
    Jan 2 at 21:59






  • 6




    $begingroup$
    For your specific case, one of the numbers you are looking to find is the final nonzero digit. As $15! = 2^{11}cdot3^6cdot5^3cdot7^2cdot11cdot13$ the final nonzero digit will be $2^8cdot3^6cdot7^2cdot11cdot13pmod{10}$ which is a much easier calculation to do modulo 10 than fully expanding out the product. $2^8cdot3^6cdot7^2cdot11cdot13equiv 6cdot 9cdot 9cdot 1cdot 3equiv 8pmod{10}$. This would be admittedly more difficult if both of the numbers were located further in the middle and not next to the trailing zeroes.
    $endgroup$
    – JMoravitz
    Jan 2 at 22:09
















6












6








6


1



$begingroup$



$$15! equiv 1cdot 2cdot 3cdot,cdots,cdot 15 equiv 1square0767436square000$$



Using a calculator, I know that the answer is $3$ and $8$, but I know that the answer can be calculated by hand.




How to calculate the missing digits? I know that large factorials can be estimated using Stirling's approximation:
$$15! approx sqrt{2picdot 15}
cdot left(frac{15}{e}right)^{15}$$

which is not feasible to calculate by hand.



The resulting number must be divisible by 9 which means that the digit sum must add up to 9 and is also divisible by 11 which means that the alternating digit sum must be divisible by 11:



$1+ d_0 + 0 + 7 +6 +7 +4 +3+6+d_1+0+0+0 mod phantom{1}9 equiv ,34 + d_0 + d_1 mod phantom{1}9 equiv 0 $
$-1+ d_0 - 0 + 7 -6 +7 -4 +3-6+d_1-0+0-0 mod 11 equiv d_0 + d_1 mod 11 equiv 0 $



The digits $3$ and $8$, or $7$ and $4$, fulfill both of the requirements.










share|cite|improve this question











$endgroup$





$$15! equiv 1cdot 2cdot 3cdot,cdots,cdot 15 equiv 1square0767436square000$$



Using a calculator, I know that the answer is $3$ and $8$, but I know that the answer can be calculated by hand.




How to calculate the missing digits? I know that large factorials can be estimated using Stirling's approximation:
$$15! approx sqrt{2picdot 15}
cdot left(frac{15}{e}right)^{15}$$

which is not feasible to calculate by hand.



The resulting number must be divisible by 9 which means that the digit sum must add up to 9 and is also divisible by 11 which means that the alternating digit sum must be divisible by 11:



$1+ d_0 + 0 + 7 +6 +7 +4 +3+6+d_1+0+0+0 mod phantom{1}9 equiv ,34 + d_0 + d_1 mod phantom{1}9 equiv 0 $
$-1+ d_0 - 0 + 7 -6 +7 -4 +3-6+d_1-0+0-0 mod 11 equiv d_0 + d_1 mod 11 equiv 0 $



The digits $3$ and $8$, or $7$ and $4$, fulfill both of the requirements.







elementary-number-theory factorial






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edited Jan 2 at 23:48







user630758

















asked Jan 2 at 21:30









DarkiceDarkice

1415




1415








  • 8




    $begingroup$
    Well, you know that the sum of the digits is divisible by $9$ and the alternating sum of the digits is divisible by $11$. That should tell you what the two digits are, though not their order. But you should be able to argue that the first non-zero digit from the right must be even.
    $endgroup$
    – lulu
    Jan 2 at 21:36










  • $begingroup$
    Why is the sum of the digits divisible by 9?
    $endgroup$
    – Darkice
    Jan 2 at 21:37






  • 3




    $begingroup$
    @Darkice because for every natural number $ngeq 6$ you have that $n!$ is a multiple of nine and a natural number is a multiple of nine if and only if the sum of its digits is a multiple of nine.
    $endgroup$
    – JMoravitz
    Jan 2 at 21:41








  • 1




    $begingroup$
    Alright, now I know that the digits must add up to a multiple of 9 and must be divisible by 11. So the digits could be 7 and 4 or 3 and 8 which would both fulfil the requirements.
    $endgroup$
    – Darkice
    Jan 2 at 21:59






  • 6




    $begingroup$
    For your specific case, one of the numbers you are looking to find is the final nonzero digit. As $15! = 2^{11}cdot3^6cdot5^3cdot7^2cdot11cdot13$ the final nonzero digit will be $2^8cdot3^6cdot7^2cdot11cdot13pmod{10}$ which is a much easier calculation to do modulo 10 than fully expanding out the product. $2^8cdot3^6cdot7^2cdot11cdot13equiv 6cdot 9cdot 9cdot 1cdot 3equiv 8pmod{10}$. This would be admittedly more difficult if both of the numbers were located further in the middle and not next to the trailing zeroes.
    $endgroup$
    – JMoravitz
    Jan 2 at 22:09
















  • 8




    $begingroup$
    Well, you know that the sum of the digits is divisible by $9$ and the alternating sum of the digits is divisible by $11$. That should tell you what the two digits are, though not their order. But you should be able to argue that the first non-zero digit from the right must be even.
    $endgroup$
    – lulu
    Jan 2 at 21:36










  • $begingroup$
    Why is the sum of the digits divisible by 9?
    $endgroup$
    – Darkice
    Jan 2 at 21:37






  • 3




    $begingroup$
    @Darkice because for every natural number $ngeq 6$ you have that $n!$ is a multiple of nine and a natural number is a multiple of nine if and only if the sum of its digits is a multiple of nine.
    $endgroup$
    – JMoravitz
    Jan 2 at 21:41








  • 1




    $begingroup$
    Alright, now I know that the digits must add up to a multiple of 9 and must be divisible by 11. So the digits could be 7 and 4 or 3 and 8 which would both fulfil the requirements.
    $endgroup$
    – Darkice
    Jan 2 at 21:59






  • 6




    $begingroup$
    For your specific case, one of the numbers you are looking to find is the final nonzero digit. As $15! = 2^{11}cdot3^6cdot5^3cdot7^2cdot11cdot13$ the final nonzero digit will be $2^8cdot3^6cdot7^2cdot11cdot13pmod{10}$ which is a much easier calculation to do modulo 10 than fully expanding out the product. $2^8cdot3^6cdot7^2cdot11cdot13equiv 6cdot 9cdot 9cdot 1cdot 3equiv 8pmod{10}$. This would be admittedly more difficult if both of the numbers were located further in the middle and not next to the trailing zeroes.
    $endgroup$
    – JMoravitz
    Jan 2 at 22:09










8




8




$begingroup$
Well, you know that the sum of the digits is divisible by $9$ and the alternating sum of the digits is divisible by $11$. That should tell you what the two digits are, though not their order. But you should be able to argue that the first non-zero digit from the right must be even.
$endgroup$
– lulu
Jan 2 at 21:36




$begingroup$
Well, you know that the sum of the digits is divisible by $9$ and the alternating sum of the digits is divisible by $11$. That should tell you what the two digits are, though not their order. But you should be able to argue that the first non-zero digit from the right must be even.
$endgroup$
– lulu
Jan 2 at 21:36












$begingroup$
Why is the sum of the digits divisible by 9?
$endgroup$
– Darkice
Jan 2 at 21:37




$begingroup$
Why is the sum of the digits divisible by 9?
$endgroup$
– Darkice
Jan 2 at 21:37




3




3




$begingroup$
@Darkice because for every natural number $ngeq 6$ you have that $n!$ is a multiple of nine and a natural number is a multiple of nine if and only if the sum of its digits is a multiple of nine.
$endgroup$
– JMoravitz
Jan 2 at 21:41






$begingroup$
@Darkice because for every natural number $ngeq 6$ you have that $n!$ is a multiple of nine and a natural number is a multiple of nine if and only if the sum of its digits is a multiple of nine.
$endgroup$
– JMoravitz
Jan 2 at 21:41






1




1




$begingroup$
Alright, now I know that the digits must add up to a multiple of 9 and must be divisible by 11. So the digits could be 7 and 4 or 3 and 8 which would both fulfil the requirements.
$endgroup$
– Darkice
Jan 2 at 21:59




$begingroup$
Alright, now I know that the digits must add up to a multiple of 9 and must be divisible by 11. So the digits could be 7 and 4 or 3 and 8 which would both fulfil the requirements.
$endgroup$
– Darkice
Jan 2 at 21:59




6




6




$begingroup$
For your specific case, one of the numbers you are looking to find is the final nonzero digit. As $15! = 2^{11}cdot3^6cdot5^3cdot7^2cdot11cdot13$ the final nonzero digit will be $2^8cdot3^6cdot7^2cdot11cdot13pmod{10}$ which is a much easier calculation to do modulo 10 than fully expanding out the product. $2^8cdot3^6cdot7^2cdot11cdot13equiv 6cdot 9cdot 9cdot 1cdot 3equiv 8pmod{10}$. This would be admittedly more difficult if both of the numbers were located further in the middle and not next to the trailing zeroes.
$endgroup$
– JMoravitz
Jan 2 at 22:09






$begingroup$
For your specific case, one of the numbers you are looking to find is the final nonzero digit. As $15! = 2^{11}cdot3^6cdot5^3cdot7^2cdot11cdot13$ the final nonzero digit will be $2^8cdot3^6cdot7^2cdot11cdot13pmod{10}$ which is a much easier calculation to do modulo 10 than fully expanding out the product. $2^8cdot3^6cdot7^2cdot11cdot13equiv 6cdot 9cdot 9cdot 1cdot 3equiv 8pmod{10}$. This would be admittedly more difficult if both of the numbers were located further in the middle and not next to the trailing zeroes.
$endgroup$
– JMoravitz
Jan 2 at 22:09












6 Answers
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$begingroup$

Another way to reason is to note that $15!$ is divisible by $2cdot4cdot2cdot8cdot2cdot4cdot2=2^{11}$, which means $1square0767436square$ is divisible by $2^8$. In particular, it's divisible by $8$. But since $8mid1000$ and $8mid360$, the final $square$ must be either $0$ or $8$. But it can't be $0$, since $15!$ has only three powers of $5$ (from $5$, $10$, and $15$), and those were already accounted for in the final three $0$'s of the number. Thus the final $square$ is an $8$. Casting out $9$'s now reveals that the first $square$ is a $3$.



Remark: It wasn't strictly necessary to determine the exact power of $2$ (namely $2^{11}$) that divides $15!$, merely that $2^6$ divides it, but it wasn't that hard to do.






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    2












    $begingroup$

    You can cast out $9$’s and $11$’s:
    begin{align}
    1+x+0+7+6+7+4+3+6+y+0+0+0&=x+y+34 \
    1-x+0-7+6-7+4-3+6-y+0-0+0&=-x-y
    end{align}

    Thus $x+y=11$ (it can't be $x=y=0$).



    Then find the remainder modulo $10000$; since
    $$
    15!=2^{11}cdot 3^6cdot 5^3cdot 7^2cdot11cdot13=1000cdot 2^8cdot3^6cdot7^2cdot 11cdot 13
    $$

    this means finding the remainder modulo $10$ of
    $$
    2^8cdot3^6cdot7^2cdot 11cdot 13
    $$

    that gives $8$ with a short computation.






    share|cite|improve this answer









    $endgroup$





















      1












      $begingroup$

      Using the divisibility rule for 7 the answer boils down to 3 and 8:



      $-368+674+307+1 mod 7 equiv 0$






      share|cite|improve this answer









      $endgroup$













      • $begingroup$
        for the order problem you could remove three zeros from the Least significant digit side and say it still needs to be divisible by 2 or 4 cause there are lots of 2's and 4's in the original multiplication, so 3 couldn't be on the right side.
        $endgroup$
        – no0ob
        Jan 2 at 23:34












      • $begingroup$
        The order is also given by the divisibilty rule for 7.
        $endgroup$
        – Darkice
        Jan 3 at 8:54



















      1












      $begingroup$

      Okay, $15! = 1*2*3..... *15=1a0767436b000$.



      Why does it end with $000$? Well, obviously because $5,10,15$ all divide into it so $5^3$ divides into it and at least three copies of $2$ divid into it so $2^3*5^3 =1000$ divide into it.



      If we divide $15!$ by $100 = 8*5^3$ we get



      $1a0767436b = 1*2*3*4*6*7*9*2*11*12*13*14*3$



      If we want to find the last digit of that we can do



      $1a0767436b equiv b pmod {10}$ and



      $1*2*3*4*6*7*9*2*11*12*13*14*3equiv 2*3*4*(-4)*(-3)*(-1)*2*1*2*3*4*3equiv$



      $-2^9*3^4 equiv -512*81equiv -2 equiv 8pmod {10}$..



      So $b = 8$.



      But what is $a$?



      Well, $11|1a0767436b$ and $9|1a0767436b$.



      So $1+0+6+4+6 - a - 7-7-3-b = 11k$ for some integer $k$. And $1+a+0+7+6+7+4+3+6+b = 9j$ for some integer $j$.



      So $-a -8 =11k$ so as $0le a le 9$ we have $a = 3$.



      And that's that $15! = 1307674368000$..... IF we assume the person who asked this question was telling the truth.



      We do know that $15!$ ends with $.... 8000$ but we are completely taking someone elses word for it that it begins with $1a0767436....$






      share|cite|improve this answer









      $endgroup$





















        1












        $begingroup$

        Let $d_1$ and $d_2$ be the two unknown digits.



        The number must be divisible by $8000$, because $15!$ contains $8$ and $1000$.



        $d_2$ is non-zero number, because $15!$ contains only three $5$s. It implies $1d_10767436d_2$ must be divisible by $8$. It implies $36d_2$ is divisible by $8$. Hence, $d_2=8$.



        Now you can use the divisibility by $9$ ($d_1+d_2=11$) and find $d_1=3$.






        share|cite|improve this answer









        $endgroup$





















          1












          $begingroup$

          $15!=2^{11}cdot 5^3cdot 7^2cdot 11cdot 13=(1000)X$ where $X=2^8cdot 3^6cdot 7^2cdot 11cdot 13.$



          The last digits of $2^8(=16^2), 3^6 (=9^3),7^2, 11,13$ are, respectively $6,9,9,1,3 .$



          Modulo $10$ we have $6cdot 9cdot 9 cdot 1cdot 3equiv 6cdot(-1)^2cdot 3equiv 18equiv 8$. So the last digit of $X$ is an $8$.



          Therefore the 2nd digit of $15!$ must be a $3$ in order for the sum of all its digits to be divisible by $9$.






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            6 Answers
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            6 Answers
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            $begingroup$

            Another way to reason is to note that $15!$ is divisible by $2cdot4cdot2cdot8cdot2cdot4cdot2=2^{11}$, which means $1square0767436square$ is divisible by $2^8$. In particular, it's divisible by $8$. But since $8mid1000$ and $8mid360$, the final $square$ must be either $0$ or $8$. But it can't be $0$, since $15!$ has only three powers of $5$ (from $5$, $10$, and $15$), and those were already accounted for in the final three $0$'s of the number. Thus the final $square$ is an $8$. Casting out $9$'s now reveals that the first $square$ is a $3$.



            Remark: It wasn't strictly necessary to determine the exact power of $2$ (namely $2^{11}$) that divides $15!$, merely that $2^6$ divides it, but it wasn't that hard to do.






            share|cite|improve this answer









            $endgroup$


















              3












              $begingroup$

              Another way to reason is to note that $15!$ is divisible by $2cdot4cdot2cdot8cdot2cdot4cdot2=2^{11}$, which means $1square0767436square$ is divisible by $2^8$. In particular, it's divisible by $8$. But since $8mid1000$ and $8mid360$, the final $square$ must be either $0$ or $8$. But it can't be $0$, since $15!$ has only three powers of $5$ (from $5$, $10$, and $15$), and those were already accounted for in the final three $0$'s of the number. Thus the final $square$ is an $8$. Casting out $9$'s now reveals that the first $square$ is a $3$.



              Remark: It wasn't strictly necessary to determine the exact power of $2$ (namely $2^{11}$) that divides $15!$, merely that $2^6$ divides it, but it wasn't that hard to do.






              share|cite|improve this answer









              $endgroup$
















                3












                3








                3





                $begingroup$

                Another way to reason is to note that $15!$ is divisible by $2cdot4cdot2cdot8cdot2cdot4cdot2=2^{11}$, which means $1square0767436square$ is divisible by $2^8$. In particular, it's divisible by $8$. But since $8mid1000$ and $8mid360$, the final $square$ must be either $0$ or $8$. But it can't be $0$, since $15!$ has only three powers of $5$ (from $5$, $10$, and $15$), and those were already accounted for in the final three $0$'s of the number. Thus the final $square$ is an $8$. Casting out $9$'s now reveals that the first $square$ is a $3$.



                Remark: It wasn't strictly necessary to determine the exact power of $2$ (namely $2^{11}$) that divides $15!$, merely that $2^6$ divides it, but it wasn't that hard to do.






                share|cite|improve this answer









                $endgroup$



                Another way to reason is to note that $15!$ is divisible by $2cdot4cdot2cdot8cdot2cdot4cdot2=2^{11}$, which means $1square0767436square$ is divisible by $2^8$. In particular, it's divisible by $8$. But since $8mid1000$ and $8mid360$, the final $square$ must be either $0$ or $8$. But it can't be $0$, since $15!$ has only three powers of $5$ (from $5$, $10$, and $15$), and those were already accounted for in the final three $0$'s of the number. Thus the final $square$ is an $8$. Casting out $9$'s now reveals that the first $square$ is a $3$.



                Remark: It wasn't strictly necessary to determine the exact power of $2$ (namely $2^{11}$) that divides $15!$, merely that $2^6$ divides it, but it wasn't that hard to do.







                share|cite|improve this answer












                share|cite|improve this answer



                share|cite|improve this answer










                answered Jan 2 at 23:47









                Barry CipraBarry Cipra

                59.4k653126




                59.4k653126























                    2












                    $begingroup$

                    You can cast out $9$’s and $11$’s:
                    begin{align}
                    1+x+0+7+6+7+4+3+6+y+0+0+0&=x+y+34 \
                    1-x+0-7+6-7+4-3+6-y+0-0+0&=-x-y
                    end{align}

                    Thus $x+y=11$ (it can't be $x=y=0$).



                    Then find the remainder modulo $10000$; since
                    $$
                    15!=2^{11}cdot 3^6cdot 5^3cdot 7^2cdot11cdot13=1000cdot 2^8cdot3^6cdot7^2cdot 11cdot 13
                    $$

                    this means finding the remainder modulo $10$ of
                    $$
                    2^8cdot3^6cdot7^2cdot 11cdot 13
                    $$

                    that gives $8$ with a short computation.






                    share|cite|improve this answer









                    $endgroup$


















                      2












                      $begingroup$

                      You can cast out $9$’s and $11$’s:
                      begin{align}
                      1+x+0+7+6+7+4+3+6+y+0+0+0&=x+y+34 \
                      1-x+0-7+6-7+4-3+6-y+0-0+0&=-x-y
                      end{align}

                      Thus $x+y=11$ (it can't be $x=y=0$).



                      Then find the remainder modulo $10000$; since
                      $$
                      15!=2^{11}cdot 3^6cdot 5^3cdot 7^2cdot11cdot13=1000cdot 2^8cdot3^6cdot7^2cdot 11cdot 13
                      $$

                      this means finding the remainder modulo $10$ of
                      $$
                      2^8cdot3^6cdot7^2cdot 11cdot 13
                      $$

                      that gives $8$ with a short computation.






                      share|cite|improve this answer









                      $endgroup$
















                        2












                        2








                        2





                        $begingroup$

                        You can cast out $9$’s and $11$’s:
                        begin{align}
                        1+x+0+7+6+7+4+3+6+y+0+0+0&=x+y+34 \
                        1-x+0-7+6-7+4-3+6-y+0-0+0&=-x-y
                        end{align}

                        Thus $x+y=11$ (it can't be $x=y=0$).



                        Then find the remainder modulo $10000$; since
                        $$
                        15!=2^{11}cdot 3^6cdot 5^3cdot 7^2cdot11cdot13=1000cdot 2^8cdot3^6cdot7^2cdot 11cdot 13
                        $$

                        this means finding the remainder modulo $10$ of
                        $$
                        2^8cdot3^6cdot7^2cdot 11cdot 13
                        $$

                        that gives $8$ with a short computation.






                        share|cite|improve this answer









                        $endgroup$



                        You can cast out $9$’s and $11$’s:
                        begin{align}
                        1+x+0+7+6+7+4+3+6+y+0+0+0&=x+y+34 \
                        1-x+0-7+6-7+4-3+6-y+0-0+0&=-x-y
                        end{align}

                        Thus $x+y=11$ (it can't be $x=y=0$).



                        Then find the remainder modulo $10000$; since
                        $$
                        15!=2^{11}cdot 3^6cdot 5^3cdot 7^2cdot11cdot13=1000cdot 2^8cdot3^6cdot7^2cdot 11cdot 13
                        $$

                        this means finding the remainder modulo $10$ of
                        $$
                        2^8cdot3^6cdot7^2cdot 11cdot 13
                        $$

                        that gives $8$ with a short computation.







                        share|cite|improve this answer












                        share|cite|improve this answer



                        share|cite|improve this answer










                        answered Jan 2 at 23:49









                        egregegreg

                        181k1485202




                        181k1485202























                            1












                            $begingroup$

                            Using the divisibility rule for 7 the answer boils down to 3 and 8:



                            $-368+674+307+1 mod 7 equiv 0$






                            share|cite|improve this answer









                            $endgroup$













                            • $begingroup$
                              for the order problem you could remove three zeros from the Least significant digit side and say it still needs to be divisible by 2 or 4 cause there are lots of 2's and 4's in the original multiplication, so 3 couldn't be on the right side.
                              $endgroup$
                              – no0ob
                              Jan 2 at 23:34












                            • $begingroup$
                              The order is also given by the divisibilty rule for 7.
                              $endgroup$
                              – Darkice
                              Jan 3 at 8:54
















                            1












                            $begingroup$

                            Using the divisibility rule for 7 the answer boils down to 3 and 8:



                            $-368+674+307+1 mod 7 equiv 0$






                            share|cite|improve this answer









                            $endgroup$













                            • $begingroup$
                              for the order problem you could remove three zeros from the Least significant digit side and say it still needs to be divisible by 2 or 4 cause there are lots of 2's and 4's in the original multiplication, so 3 couldn't be on the right side.
                              $endgroup$
                              – no0ob
                              Jan 2 at 23:34












                            • $begingroup$
                              The order is also given by the divisibilty rule for 7.
                              $endgroup$
                              – Darkice
                              Jan 3 at 8:54














                            1












                            1








                            1





                            $begingroup$

                            Using the divisibility rule for 7 the answer boils down to 3 and 8:



                            $-368+674+307+1 mod 7 equiv 0$






                            share|cite|improve this answer









                            $endgroup$



                            Using the divisibility rule for 7 the answer boils down to 3 and 8:



                            $-368+674+307+1 mod 7 equiv 0$







                            share|cite|improve this answer












                            share|cite|improve this answer



                            share|cite|improve this answer










                            answered Jan 2 at 23:15









                            DarkiceDarkice

                            1415




                            1415












                            • $begingroup$
                              for the order problem you could remove three zeros from the Least significant digit side and say it still needs to be divisible by 2 or 4 cause there are lots of 2's and 4's in the original multiplication, so 3 couldn't be on the right side.
                              $endgroup$
                              – no0ob
                              Jan 2 at 23:34












                            • $begingroup$
                              The order is also given by the divisibilty rule for 7.
                              $endgroup$
                              – Darkice
                              Jan 3 at 8:54


















                            • $begingroup$
                              for the order problem you could remove three zeros from the Least significant digit side and say it still needs to be divisible by 2 or 4 cause there are lots of 2's and 4's in the original multiplication, so 3 couldn't be on the right side.
                              $endgroup$
                              – no0ob
                              Jan 2 at 23:34












                            • $begingroup$
                              The order is also given by the divisibilty rule for 7.
                              $endgroup$
                              – Darkice
                              Jan 3 at 8:54
















                            $begingroup$
                            for the order problem you could remove three zeros from the Least significant digit side and say it still needs to be divisible by 2 or 4 cause there are lots of 2's and 4's in the original multiplication, so 3 couldn't be on the right side.
                            $endgroup$
                            – no0ob
                            Jan 2 at 23:34






                            $begingroup$
                            for the order problem you could remove three zeros from the Least significant digit side and say it still needs to be divisible by 2 or 4 cause there are lots of 2's and 4's in the original multiplication, so 3 couldn't be on the right side.
                            $endgroup$
                            – no0ob
                            Jan 2 at 23:34














                            $begingroup$
                            The order is also given by the divisibilty rule for 7.
                            $endgroup$
                            – Darkice
                            Jan 3 at 8:54




                            $begingroup$
                            The order is also given by the divisibilty rule for 7.
                            $endgroup$
                            – Darkice
                            Jan 3 at 8:54











                            1












                            $begingroup$

                            Okay, $15! = 1*2*3..... *15=1a0767436b000$.



                            Why does it end with $000$? Well, obviously because $5,10,15$ all divide into it so $5^3$ divides into it and at least three copies of $2$ divid into it so $2^3*5^3 =1000$ divide into it.



                            If we divide $15!$ by $100 = 8*5^3$ we get



                            $1a0767436b = 1*2*3*4*6*7*9*2*11*12*13*14*3$



                            If we want to find the last digit of that we can do



                            $1a0767436b equiv b pmod {10}$ and



                            $1*2*3*4*6*7*9*2*11*12*13*14*3equiv 2*3*4*(-4)*(-3)*(-1)*2*1*2*3*4*3equiv$



                            $-2^9*3^4 equiv -512*81equiv -2 equiv 8pmod {10}$..



                            So $b = 8$.



                            But what is $a$?



                            Well, $11|1a0767436b$ and $9|1a0767436b$.



                            So $1+0+6+4+6 - a - 7-7-3-b = 11k$ for some integer $k$. And $1+a+0+7+6+7+4+3+6+b = 9j$ for some integer $j$.



                            So $-a -8 =11k$ so as $0le a le 9$ we have $a = 3$.



                            And that's that $15! = 1307674368000$..... IF we assume the person who asked this question was telling the truth.



                            We do know that $15!$ ends with $.... 8000$ but we are completely taking someone elses word for it that it begins with $1a0767436....$






                            share|cite|improve this answer









                            $endgroup$


















                              1












                              $begingroup$

                              Okay, $15! = 1*2*3..... *15=1a0767436b000$.



                              Why does it end with $000$? Well, obviously because $5,10,15$ all divide into it so $5^3$ divides into it and at least three copies of $2$ divid into it so $2^3*5^3 =1000$ divide into it.



                              If we divide $15!$ by $100 = 8*5^3$ we get



                              $1a0767436b = 1*2*3*4*6*7*9*2*11*12*13*14*3$



                              If we want to find the last digit of that we can do



                              $1a0767436b equiv b pmod {10}$ and



                              $1*2*3*4*6*7*9*2*11*12*13*14*3equiv 2*3*4*(-4)*(-3)*(-1)*2*1*2*3*4*3equiv$



                              $-2^9*3^4 equiv -512*81equiv -2 equiv 8pmod {10}$..



                              So $b = 8$.



                              But what is $a$?



                              Well, $11|1a0767436b$ and $9|1a0767436b$.



                              So $1+0+6+4+6 - a - 7-7-3-b = 11k$ for some integer $k$. And $1+a+0+7+6+7+4+3+6+b = 9j$ for some integer $j$.



                              So $-a -8 =11k$ so as $0le a le 9$ we have $a = 3$.



                              And that's that $15! = 1307674368000$..... IF we assume the person who asked this question was telling the truth.



                              We do know that $15!$ ends with $.... 8000$ but we are completely taking someone elses word for it that it begins with $1a0767436....$






                              share|cite|improve this answer









                              $endgroup$
















                                1












                                1








                                1





                                $begingroup$

                                Okay, $15! = 1*2*3..... *15=1a0767436b000$.



                                Why does it end with $000$? Well, obviously because $5,10,15$ all divide into it so $5^3$ divides into it and at least three copies of $2$ divid into it so $2^3*5^3 =1000$ divide into it.



                                If we divide $15!$ by $100 = 8*5^3$ we get



                                $1a0767436b = 1*2*3*4*6*7*9*2*11*12*13*14*3$



                                If we want to find the last digit of that we can do



                                $1a0767436b equiv b pmod {10}$ and



                                $1*2*3*4*6*7*9*2*11*12*13*14*3equiv 2*3*4*(-4)*(-3)*(-1)*2*1*2*3*4*3equiv$



                                $-2^9*3^4 equiv -512*81equiv -2 equiv 8pmod {10}$..



                                So $b = 8$.



                                But what is $a$?



                                Well, $11|1a0767436b$ and $9|1a0767436b$.



                                So $1+0+6+4+6 - a - 7-7-3-b = 11k$ for some integer $k$. And $1+a+0+7+6+7+4+3+6+b = 9j$ for some integer $j$.



                                So $-a -8 =11k$ so as $0le a le 9$ we have $a = 3$.



                                And that's that $15! = 1307674368000$..... IF we assume the person who asked this question was telling the truth.



                                We do know that $15!$ ends with $.... 8000$ but we are completely taking someone elses word for it that it begins with $1a0767436....$






                                share|cite|improve this answer









                                $endgroup$



                                Okay, $15! = 1*2*3..... *15=1a0767436b000$.



                                Why does it end with $000$? Well, obviously because $5,10,15$ all divide into it so $5^3$ divides into it and at least three copies of $2$ divid into it so $2^3*5^3 =1000$ divide into it.



                                If we divide $15!$ by $100 = 8*5^3$ we get



                                $1a0767436b = 1*2*3*4*6*7*9*2*11*12*13*14*3$



                                If we want to find the last digit of that we can do



                                $1a0767436b equiv b pmod {10}$ and



                                $1*2*3*4*6*7*9*2*11*12*13*14*3equiv 2*3*4*(-4)*(-3)*(-1)*2*1*2*3*4*3equiv$



                                $-2^9*3^4 equiv -512*81equiv -2 equiv 8pmod {10}$..



                                So $b = 8$.



                                But what is $a$?



                                Well, $11|1a0767436b$ and $9|1a0767436b$.



                                So $1+0+6+4+6 - a - 7-7-3-b = 11k$ for some integer $k$. And $1+a+0+7+6+7+4+3+6+b = 9j$ for some integer $j$.



                                So $-a -8 =11k$ so as $0le a le 9$ we have $a = 3$.



                                And that's that $15! = 1307674368000$..... IF we assume the person who asked this question was telling the truth.



                                We do know that $15!$ ends with $.... 8000$ but we are completely taking someone elses word for it that it begins with $1a0767436....$







                                share|cite|improve this answer












                                share|cite|improve this answer



                                share|cite|improve this answer










                                answered Jan 3 at 0:53









                                fleabloodfleablood

                                69.4k22685




                                69.4k22685























                                    1












                                    $begingroup$

                                    Let $d_1$ and $d_2$ be the two unknown digits.



                                    The number must be divisible by $8000$, because $15!$ contains $8$ and $1000$.



                                    $d_2$ is non-zero number, because $15!$ contains only three $5$s. It implies $1d_10767436d_2$ must be divisible by $8$. It implies $36d_2$ is divisible by $8$. Hence, $d_2=8$.



                                    Now you can use the divisibility by $9$ ($d_1+d_2=11$) and find $d_1=3$.






                                    share|cite|improve this answer









                                    $endgroup$


















                                      1












                                      $begingroup$

                                      Let $d_1$ and $d_2$ be the two unknown digits.



                                      The number must be divisible by $8000$, because $15!$ contains $8$ and $1000$.



                                      $d_2$ is non-zero number, because $15!$ contains only three $5$s. It implies $1d_10767436d_2$ must be divisible by $8$. It implies $36d_2$ is divisible by $8$. Hence, $d_2=8$.



                                      Now you can use the divisibility by $9$ ($d_1+d_2=11$) and find $d_1=3$.






                                      share|cite|improve this answer









                                      $endgroup$
















                                        1












                                        1








                                        1





                                        $begingroup$

                                        Let $d_1$ and $d_2$ be the two unknown digits.



                                        The number must be divisible by $8000$, because $15!$ contains $8$ and $1000$.



                                        $d_2$ is non-zero number, because $15!$ contains only three $5$s. It implies $1d_10767436d_2$ must be divisible by $8$. It implies $36d_2$ is divisible by $8$. Hence, $d_2=8$.



                                        Now you can use the divisibility by $9$ ($d_1+d_2=11$) and find $d_1=3$.






                                        share|cite|improve this answer









                                        $endgroup$



                                        Let $d_1$ and $d_2$ be the two unknown digits.



                                        The number must be divisible by $8000$, because $15!$ contains $8$ and $1000$.



                                        $d_2$ is non-zero number, because $15!$ contains only three $5$s. It implies $1d_10767436d_2$ must be divisible by $8$. It implies $36d_2$ is divisible by $8$. Hence, $d_2=8$.



                                        Now you can use the divisibility by $9$ ($d_1+d_2=11$) and find $d_1=3$.







                                        share|cite|improve this answer












                                        share|cite|improve this answer



                                        share|cite|improve this answer










                                        answered Jan 3 at 4:55









                                        farruhotafarruhota

                                        19.9k2738




                                        19.9k2738























                                            1












                                            $begingroup$

                                            $15!=2^{11}cdot 5^3cdot 7^2cdot 11cdot 13=(1000)X$ where $X=2^8cdot 3^6cdot 7^2cdot 11cdot 13.$



                                            The last digits of $2^8(=16^2), 3^6 (=9^3),7^2, 11,13$ are, respectively $6,9,9,1,3 .$



                                            Modulo $10$ we have $6cdot 9cdot 9 cdot 1cdot 3equiv 6cdot(-1)^2cdot 3equiv 18equiv 8$. So the last digit of $X$ is an $8$.



                                            Therefore the 2nd digit of $15!$ must be a $3$ in order for the sum of all its digits to be divisible by $9$.






                                            share|cite|improve this answer









                                            $endgroup$


















                                              1












                                              $begingroup$

                                              $15!=2^{11}cdot 5^3cdot 7^2cdot 11cdot 13=(1000)X$ where $X=2^8cdot 3^6cdot 7^2cdot 11cdot 13.$



                                              The last digits of $2^8(=16^2), 3^6 (=9^3),7^2, 11,13$ are, respectively $6,9,9,1,3 .$



                                              Modulo $10$ we have $6cdot 9cdot 9 cdot 1cdot 3equiv 6cdot(-1)^2cdot 3equiv 18equiv 8$. So the last digit of $X$ is an $8$.



                                              Therefore the 2nd digit of $15!$ must be a $3$ in order for the sum of all its digits to be divisible by $9$.






                                              share|cite|improve this answer









                                              $endgroup$
















                                                1












                                                1








                                                1





                                                $begingroup$

                                                $15!=2^{11}cdot 5^3cdot 7^2cdot 11cdot 13=(1000)X$ where $X=2^8cdot 3^6cdot 7^2cdot 11cdot 13.$



                                                The last digits of $2^8(=16^2), 3^6 (=9^3),7^2, 11,13$ are, respectively $6,9,9,1,3 .$



                                                Modulo $10$ we have $6cdot 9cdot 9 cdot 1cdot 3equiv 6cdot(-1)^2cdot 3equiv 18equiv 8$. So the last digit of $X$ is an $8$.



                                                Therefore the 2nd digit of $15!$ must be a $3$ in order for the sum of all its digits to be divisible by $9$.






                                                share|cite|improve this answer









                                                $endgroup$



                                                $15!=2^{11}cdot 5^3cdot 7^2cdot 11cdot 13=(1000)X$ where $X=2^8cdot 3^6cdot 7^2cdot 11cdot 13.$



                                                The last digits of $2^8(=16^2), 3^6 (=9^3),7^2, 11,13$ are, respectively $6,9,9,1,3 .$



                                                Modulo $10$ we have $6cdot 9cdot 9 cdot 1cdot 3equiv 6cdot(-1)^2cdot 3equiv 18equiv 8$. So the last digit of $X$ is an $8$.



                                                Therefore the 2nd digit of $15!$ must be a $3$ in order for the sum of all its digits to be divisible by $9$.







                                                share|cite|improve this answer












                                                share|cite|improve this answer



                                                share|cite|improve this answer










                                                answered Jan 3 at 10:16









                                                DanielWainfleetDanielWainfleet

                                                34.8k31648




                                                34.8k31648






























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