Theorem 3.3.5. Permutations when not all items are distinguishable (Multinomial Coefficients).
If n items belong to s categories, \(n_1\) in first, \(n_2\) in second, ... , \(n_s\) in the last, the number of ways to pick all is
\begin{equation*}
\frac{n!}{n_1! \cdot n_2! ... n_s!}
\end{equation*}