Lary, Mo and Jo are standing on one side of a lake. They have 4 boats that they can use. Each boat is a different color and moves at a different speed. The red boat can cross the lake in 2 min, the blue boat in 3 min, the green boat in 7 min and the yellow in 10 min. They challenge eachother to a contest. The goal is for one person to get all four boats to the other side of the lake in 21 minutes. Here are the rules: 1. The only way to get from one side of the lake to the other is by driving the boats- walking around the lake is not allowed. 2. Each boat can be driven across the lake individually or by towing another boat. 3. Towing more than one boat at a time is not allowed. If a boat is towed, the time to cross the lake is determined by the speed of the slowest boat. For example, if the red boat (2 min) tows the green boat(7 min) it will take 7 minutes to cross the lake. Mo says he can do it in the 21 minutes. How does he do it?