Problem of the Week

Many thanks to Mary Jane Sterling for suggesting the following problem.

There are 21 seats in a long narrow boat; men are sitting in the first 10 seats, women are sitting in the last 10 seats, and the seat in the middle is initially empty.  The men and women want to change positions.  In order not to tip the boat, they may make only two types of moves: (1) any person sitting next to the empty seat can slide over into it, or (2) any person can step over one other person and into the empty seat. 

Can they change positions so that all 10 women are in the front of the boat and all 10 men are in the rear, and if so, how many moves will it take them?

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ã2002 Alberto L. Delgado