## Precalculus (6th Edition) Blitzer

We know that the order in which the five planes can line up for departure on a runway matters. Thus, we use permutations. Here, $n=5,r=5$. \begin{align} & _{5}{{P}_{5}}=\frac{5!}{\left( 5-5 \right)!} \\ & =\frac{5!}{0!} \\ & =\frac{5\times 4\times 3\times 4\times 1!}{1} \\ & =120 \end{align}