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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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My answer rationalized | Comment 5 of 7 |
I guess I took too simple an approach.  I think the chance that my answer (8) is correct is about 1 in 4.  

I will decline to elaborate on my flawed thinking, except to give one indication of where I went wrong.  Because multiplying by 2 and 5 does not change the last digit, I took the leap of inferring (incorrectly) that multiplying by 12 and 15 would also not change the last-non-zero digit.  This simplified the problem significantly. :-)

  Posted by Steve Herman on 2012-02-06 17:54:52
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