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smc

number theory senior

Problem

Determine the largest positive integer such that is divisible by .
(A)
(B)
(C)
(D)
(E)
Solution
We first observe that since there will be more 2s than 5s in , we are looking for the largest such that divides . We will use the fact that: (This is an application of Legendre's formula). From and onwards, . Thus, our calculation becomes Since none of the answer choices equal 250, the answer is .
Final answer
E