Will calculate the permutations for P(n,r) = n! / (n - r)!. "The number of ways of obtaining an ordered subset of r elements from a set of n elements."
For r <= n, n >= 0, and r >= 0.
P(n,r) = n! / (n - r)!
 For more information on permutations and combinations please see http://mathworld.wolfram.com/Permutation.html