Thursday, March 14, 2019

elementary number theory - Is there a way to figure out the n such that n! ends with exactly k zeroes?

Given n, I can find the number of zeroes at the end of the decimal representation of n! by i=1n5i.


Is there a way to reverse this? That is, given k, is there a way to find out how many n exist such that n! has exactly k zeroes at the end of its decimal representation besides making educated guesses and checking them?

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