What Is Permutation?
A permutation counts how many different ordered arrangements are possible when choosing r items from a set of n, where the order of selection matters. It's closely related to a combination, which counts the same kind of selection but ignores order. Permutations are always greater than or equal to the corresponding combination, since each combination can usually be arranged in more than one order.
Example
Choosing and ordering 2 winners (1st and 2nd place) from 5 runners gives 5 × 4 = 20 permutations, since there are 5 choices for 1st place and 4 remaining choices for 2nd.