Chapter 10 Counting Methods and Probability - 10.1 Apply the counting Principles and Permutations - 10.1 Exercises - Skill Practice - Page 686: 2

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We know the following equation for permutations: $$nP_r=\frac{n!}{(n-r)!}$$ Thus, we find: $$nP_r=\frac{n!}{(n-0)!}=1$$ This makes sense because when $r$ is 0, there are 0 objects from the group, so there is only one way to organize them.