Hello,
Should we multiply by k or n%k if (n - i*k) equals k or n%k ?
Which sample is correct?
Sample 1
8!! = 8 *(8-2)*(8-4)*(8-6)*2

Sample 2
8!! = 8 *(8-2)*(8-4)*(8-6)

Sample 2 is correct

N! = 1 x 2 x 3 x 4....x (N-2) x (N-1) x N
N! = (N-1)! x N
factorial(N) = factorial(N-1) x N
We can calculate factorial of a number using recursion using above formulae.
http://www.techcrashcourse.com/2014/10/c-program-find-factorial-of-number.html
http://www.techcrashcourse.com/2015/03/c-program-to-find-factorial-of-number-using-recursion.html
Hope it helps.