Sum of digits of a factorial

Question

Link to the original problem

It\'s not a homework question. I just thought that someone might know a real solution to this problem.

I was on

Answer 1

You have to compute the fatcorial.

1 * 2 * 3 * 4 * 5 = 120.

If you only want to calculate the sum of digits, you can ignore the ending zeroes.

For 6! you can do 12 x 6 = 72 instead of 120 * 6

For 7! you can use (72 * 7) MOD 10

EDIT.

I wrote a response too quickly...

10 is the result of two prime numbers 2 and 5.

Each time you have these 2 factors, you can ignore them.

1 * 2 * 3 * 4 * 5 * 6 * 7 * 8 * 9 * 10 * 11 * 12 * 13 * 14 * 15...

1   2   3   2   5   2   7   2   3    2   11    2   13    2    3
            2       3       2   3    5         2         7    5
                            2                  3

The factor 5 appears at 5, 10, 15...
Then a ending zero will appear after multiplying by 5, 10, 15...

We have a lot of 2s and 3s... We'll overflow soon :-(

Then, you still need a library for big numbers.

I deserve to be downvoted!