Find the number of ordered quintuples of DISTINCT digits 0-9 ( a , b , c , d , e ) such that a + b + c + d + e is a multiple of 5.
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There are 52 ways to choose a set A ⊂ { 0 , … , 9 } in order that ∣ A ∣ = 5 and ∑ A ≡ 0 ( m o d 5 ) , since, once set ξ = exp ( 5 2 π i ) : 5 1 k = 0 ∑ 4 [ y 5 ] j = 0 ∏ 9 ( 1 + y ( x ξ k ) j ) = 5 2 , so the answer is just 5 2 ⋅ 5 ! = 6 2 4 0 .
Can you explain your solution with words instead of mathematical symbols?
It looks like a generating function, but which one? and what is the x in there?
I'm intrigued by this solution. Can you provide a reference? What theorem is this based on?
Can you explain your solution in more detail? Specifically, what is the equation doing / how does the counting work?
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Yea, it was confusing to me as well. I just bashed cases in mod 5 ie 01234 gives 32*120 cases
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conta=6240 :D
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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