$n!$ 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 $100!$.
This is pretty similar to Problem 16. We'll just use BigInt to compute large factorials, convert them to strings, and sum up the digits.
function sum_of_factorial_digits(n)
fact = factorial(big(n))
digits_sum = sum(parse(Int, c) for c in string(fact))
return digits_sum
end
sum_of_factorial_digits(100) computes the sum of the digits of $100!$ in 962.000 ns.
sum_of_factorial_digits(1000000) computes the sum of the digits of $10^6!$ to be 23903442 in 522.110 ms.