each valve a, ba,b, and cc, when open, releases water into a tank at its own constant rate. with all three valves open, the tank fills in 1 hour, with only valves aa and cc open it takes 1.5 hours, and with only valves bb and cc open it takes 2 hours. how many hours will it take to fill the tank with only valves aa and bb open? express your answer as a decimal number.
