nCr = n! ÷ (n - r)!r!
6C3 = 6! ÷ (6 - 3)!3!
6C3 = 6! ÷ 3!3!
6C3 = 6•5•4•3! ÷ 3!3!
6C3 = 6•5•4 ÷ 3!
6C3 = 5•4
6C3 = 20✔
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