TITLE: How many 0's are at the end of 20!
QUESTION [5 upvotes]: I'm not exactly sure how to answer this question, any help would be appreciated. After reading this I'm still not sure.
Cheers

REPLY [2 votes]: General formula (for the interested) about the number of zeroes in n! in any base (b). First consider all prime factors of b, then consider the biggest one (p). Then use this formula.
$\lfloor n/p \rfloor$ + $\lfloor n/p^2 \rfloor$ + $\lfloor n/p^3 \rfloor$ + ....
This and using the fact that, the floor becomes zero after some exponent, you can calculate the number of zeroes in any base.
http://maths-on-line.blogspot.in/