Permutation calculator Formula:
   P(n,r) = n! / (n - r)!
Where:
   n: n elements (n > 0)
   r: r subset elements (0 < r <= n)

Combination calculator Formula:
   C(n,r) = n! /(r! * (n - r)!)
Where:
   n: n elements (n > 0)
   r: r subset elements (0 < r <= n)