Permutations

A permutation is a linear arrangement of a group of items. Each unique way of ordering the items of a group is known as a permutation. The number of ways to arrange a group of n items is n! (factorial of n). This means n x n-1 x n-2 x ... x 1.

The number of permutations (of length r) that can be made from a set of n items is:

n! / (n-r)!

Combinations

A combination is a set containing a certain number of objects that have been selected from another set. In combinations, the order of the elements does not matter.

The number of combinations (of length r) that can be made from a set of n items is:

n! / ((n-r)! r!)

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