Theorem 3.4.1. Combinations of a subset without replacement.
The number of ways to select an unordered group of r items from a set of n distinct items is
\begin{equation*}
_nC_r = \frac{n!}{r!(n-r)!}
\end{equation*}
This is sometimes denoted C(n,r) or \(C_r^n\) or \({n \choose r}\text{.}\)