## Precalculus: Concepts Through Functions, A Unit Circle Approach to Trigonometry (3rd Edition)

In the first place any of the 5 books can be placed, but in the second place we can only choose one from the $4$remaining books. The number of choices for the next place will also be one less than the previous one. . Thus, this involves a permutation where $n=5$ and $r =5$. Using the formula $P(n,n) = n!$ gives: $P(5,5) = 5! = 5(4)(3)(2)(1)=120$