Theorem 3.3.2. Permutations of a subset without replacement.
The number of ways to arrange r items from a set of n distinct items is
\begin{equation*}
_nP_r = \frac{n!}{(n-r)!}
\end{equation*}
This is sometimes denoted also as P(n,r) or \(P_r^n\text{.}\)