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 nonzero 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 20120206 13:20:14 