text: b) Why there are four masses here 1 small guy + 3 big guy = 4.  Divide 4 into 20, you get 5.  But that is only half the answer.  The big guy's mass is an anchor for the small guy.  So every time the big guy pulls he just pulling the small guy closer and every time the small guy pulls he is moving closer in a ratio of 3 to 1.  So the big guy only moves 5m and the small guy move 15m.