A permutation is an ordered grouping of a set. The most important thing to remember about permutations is that order does matter.
permutation graphic

The variable n is the total number of possible choices, and r is the number chosen.
In permutations, order matters. If first place and third place changed with each other and the rest of the order stayed the same, it would still be considered a different arrangement. This is important in order to distinguish between permutations and combinations

There are 60 different permutations of medal winners, or 60 different sets of 3 that would win either gold, silver, or bronze medals

Here is a script which does the math for you.
Enter the number (n) of items involved and the the number of items to be combined (c):
n=P =r

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©Jeff LeMieux, revised 5/2013