Divisibility comparison between factorials

Problem 383

Published on 05 May 2012 at 11:00 pm [Server Time]

Let f5(n) be the largest integer x for which 5x divides n.
For example, f5(625000) = 7.

Let T5(n) be the number of integers i which satisfy f5((2·i-1)!) < 2·f5(i!) and 1 ≤ i ≤ n.
It can be verified that T5(103) = 68 and T5(109) = 2408210.

Find T5(1018).

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