Permutations are the number of possible orders for a given group of elements. You can determine the permutation by multiplying by the number of possible choices for each selection.
For example, let's say you are given 4 distinct elements, and want to know how many ways you can arrange 3 of them. You can pick from 4 elements for the first choice, 3 for the second, and 2 for the third:
4 Permute 3 = 4 * 3 * 2 = 24 possible ways
(See below for additional examples.)
The general Permutation formula for a permute b is:
a * (a-1) * (a-2)... (until you've multiplied b elements.)
Can you calculate a permute b?