Answer
The number of ways five planes can be lined up for departure is: 120.
Work Step by Step
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$.
$\begin{align}
& _{5}{{P}_{5}}=\frac{5!}{\left( 5-5 \right)!} \\
& =\frac{5!}{0!} \\
& =\frac{5\times 4\times 3\times 4\times 1!}{1} \\
& =120
\end{align}$
