Permutation (P(n,r)): The number of ways to choose r elements from a set of n elements

P(n,r) = n! / (n-r)!

Example:

• P(5,3) = 5! / (5-3)! = 5! / 2! = (5 × 4 × 3) = 60
• P(4,4) = 4! / (4-4)! = 4! / 0! = 24

Note: n ≥ r and n, r ≥ 0