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Problem 20 : Factorial Digit Sum

$n!$ means $n \times (n - 1) \times \cdots \times 3 \times 2 \times 1$.

For example, $10! = 10 \times 9 \times \cdots \times 3 \times 2 \times 1 = 3628800$,
and the sum of the digits in the number $10!$ is $3 + 6 + 2 + 8 + 8 + 0 + 0 = 27$.

Find the sum of the digits in the number $100!$.

Expected Output

Published:	Friday, 21st June 2002, 12:00 pm
Difficulty rating:	5%
Overview (PDF):	problem 20
Forum problem:	problem 20