## Factorial digit sum: Project Euler Problem 20

$n!$ means $n\times(n-1)\times...\times2\times1$
For example, $10! = 10 \times 9 \times ... \times 3 \times 2 \times 1 = 3628800$,
and the sum of the digits in the number 10! is $3 + 6 + 2 + 8 + 8 + 0 + 0 = 27$.