Reading from left to right, what is the last non - zero digit in ((20!)!)!? (That is, the factorial of factorial of 20 factorial).

I'd like to know how either answer was found.

I found a couple of algorithms for computing the last non-zero digit of n! but they require writing n in base 5.
(20!)! has over 10^19 digits in base 10, even more in base 5. 

I can't help but think this problem is harder than d4.

Posted by Jer on 2012-02-06 13:20:14