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Possible Duplicate:
How come the number $N!$ can terminate in exactly $1,2,3,4,$ or $6$ zeroes but never $5$ zeroes?

I know that I have to find the number of factors of $5$'s, $25$'s, $125$'s etc. in order to do this. But how can you derive such a formula for any number $n$?

  • 1
    [This](http://www.purplemath.com/modules/factzero.htm) and [this](http://math.stackexchange.com/questions/18261/number-of-zeros-not-possible-in-n).2012-02-20

2 Answers 2