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With permutations, we made a formula involving k ! ( n − k ) ! n ! with n and k being the number of ways of choosing k things from a collection of n things. If there was a problem that if there was 2 people, how many handshakes could be made? when n = k = 2 . Then the equation will be 2 ! ( 0 ) ! 2 ! = 1 . That means that 0 ! must be 1 for the equation to work. □