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A combination of k elements from set of n elements is subset of k elements from this set. Order doesn't matter. The number of these combinations is given by formula n_C_k = (n over k). Examples. Combinations of 2 elements from set of 3 elements a, b, c: {a, b}, {a, c}, {b, c}. 3_C_2 = (3 over 2) = 3! / 2!(3-2)! = 2!⋅4 / 1!⋅2! = 2. Combinations of 3 elements from set of 4 elements a, b, c, d: {a, b, c}, {a, b, d}, {a, c, d}, {b, c, d} 4_C_3 = (4 over 3) = 4! / 3!(4-3)! = 3!⋅4 / 1!⋅3! = 4.