Can you count?

Find the number of ordered quintuples of DISTINCT digits 0-9 ( a , b , c , d , e ) (a,b,c,d,e) such that a + b + c + d + e a+b+c+d+e is a multiple of 5.


The answer is 6240.

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3 solutions

Daniel Liu
Jun 24, 2014

For convenience, we can have the numbers 0, 1, 2, ..., 9 instead be expressed as 0,0,1,1,...,4,4 in mod 5. Then we can bash super hard to get the following cases: 01234, 00113, 00221, 00334, 00442, 11224, 11332, 11440, 22330, 22443, 33441. The first case has 5!x32 ways, because there are 5! ways to order it, and also we must multiply by 2^5 because either 0 or 5 can be 0. either 1 or 6 can be 1, etc etc. The next 10 cases each have 5!x2 ways. So the answer is 5!x52=6240

Jack D'Aurizio
Apr 23, 2014

There are 52 ways to choose a set A { 0 , , 9 } A\subset\{0,\ldots,9\} in order that A = 5 |A|=5 and A 0 ( m o d 5 ) \sum A\equiv 0\pmod{5} , since, once set ξ = exp ( 2 π i 5 ) \xi=\exp\left(\frac{2\pi i}{5}\right) : 1 5 k = 0 4 [ y 5 ] j = 0 9 ( 1 + y ( x ξ k ) j ) = 52 , \frac{1}{5}\sum_{k=0}^{4}[y^5]\prod_{j=0}^{9}(1+ y (x \xi^k)^j) = 52, so the answer is just 52 5 ! = 6240 52\cdot 5! = 6240 .

Can you explain your solution with words instead of mathematical symbols?

Nathan Ramesh - 7 years, 1 month ago

It looks like a generating function, but which one? and what is the x x in there?

Emanuel Sygal - 6 years, 10 months ago

I'm intrigued by this solution. Can you provide a reference? What theorem is this based on?

Igmut Schnoll - 6 years, 8 months ago

Can you explain your solution in more detail? Specifically, what is the equation doing / how does the counting work?

Calvin Lin Staff - 7 years, 1 month ago

Log in to reply

Yea, it was confusing to me as well. I just bashed cases in mod 5 ie 01234 gives 32*120 cases

Nathan Ramesh - 7 years, 1 month ago
Drop TheProblem
Sep 27, 2014
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An alternative easy solution :D

#include<iostream>
#include<stdio.h>
using namespace std;
int main()
{int conta=0;
for(int a=0;a<10;a++)
   for(int b=0;b<10;b++)
      for(int c=0;c<10;c++)
         for(int d=0;d<10;d++)
            for(int e=0;e<10;e++)
               if((a+b+c+d+e)%5==0 && a!=b && a!=c && a!=d && a!=e && b!=c && b!=d && b!=e && c!=d && c!=e && d!=e) conta++;
               cout<<conta;
 system("pause");
return 0;
} 

conta=6240 :D

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