# Chapter 11 - Counting Methods and Probability Theory - 11.2 Permutations - Exercise Set 11.2 - Page 701: 57

A permutation is an arrangement of r items taken from from a group of n items and no item is used more than once and the order of arrangement makes a difference.

#### Work Step by Step

The number of permutations possible if $r$ items are taken from $n$ items is ${}_{n}P_{r}=\displaystyle \frac{n!}{(n-r)!}$.

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