A combination in combinatorics is a selection of k elements from n, where order does not matter.
Number of combinations (without repetition)
$$C_{n}^{k}=\frac{n!}{k!(n-k)!}$$
where,
n— number of elements;
k— number of selected elements.
Number of combinations (with repetition)
$$\overline{C}_{n}^{k}=\binom{n+k-1}{k}$$
where,
n— number of elements;
k— number of selected elements.
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