Maybe you've had an experience like this. One day while canoeing with my grandkids, I tried to climb out of the canoe onto the dock. As I stepped out of the canoe, it moved off in the opposite directions, dumping me in the lake. Great screams of laughter all around. Suppose I (of mass 90 kg) started 2m from the midpoint of the canoe (mass 30 kg) and walked 4 m along the canoe toward the opposite end. How far through the water would the canoe move? The answer is 3m according to the book, but I can't figure out how they got to this.

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