Gayle runs at a speed of 4.15 m/s and dives on a sled, initially at rest on the top of a frictionless snow-covered hill. After she has descended a vertical distance of 5.00 m, her brother, who is initially at rest, hops on her back and together they continue down the hill. What is their speed at the bottom of the hill if the total vertical drop is 15.0 m? Gayle's mass is 48.5 kg, the sled has a mass of 5.50 kg and her brother has a mass of 30.0 kg.