Zoe is packing her belongings and moving in with Margaret. Her books are in boxes, which she plans to float down the river to Margaret’s shack on a square raft that is 3 m on each side and 11 cm thick. It is made of wood having a specific gravity of 0.6. If each box has a mass of 20 kg, how many boxes can be placed on the raft if the books are to remain dry? Assume that the water remains calm.