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Non - Zero Digit II (Posted on 2012-02-05) Difficulty: 4 of 5
Reading from left to right, what is the last non - zero digit in ((20!)!)!? (That is, the factorial of factorial of 20 factorial).

No Solution Yet Submitted by K Sengupta    
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Question The previous two answers | Comment 3 of 7 |
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
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