You can see a very clear explanation on permutation and combination on mathsisfun page.
Following is a sample implementation of nCr.
static long Combination(int n, int m)
{
if (m > n - m) m = n - m;
long ans = 1;
// n!/(m!(n-m)!)
for (int i = 0; i < m; i++) ans = ans * (n - i) / (i + 1);
return ans;
}?
