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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
\({n \choose r}\text{.}\)
in-context