With a drain running at its normal constant rate, Pump A alone fills an empty tank in 10 hours, Pump B alone fills it in 15 hours, and both pumps together fill it in 5 hours. In a later run, the tank starts one-fourth full. Pump A runs alone for 2 hours, then Pump B runs alone for 3 hours, and then both pumps run until the tank is full. Once the tank first reaches one-half full, the drain rate doubles and remains doubled.