Two pipes A and B can alone fill a tank in 6 hours and 9 hours respectively. Pipe C can alone empty the same tank in 6 hours. If all three pipes are opened together, then in how many hours will the tank be filled?
Tank filled by A in 1 hour = 1/6
Tank filled by B in 1 hour = 1/9
Tank emptied by C in 1 hour = 1/6
Tank filled in 1 hour when all 3 are opened = (1/6) + (1/9) - (1/6) = 1/9
∴ Tank will be filled in 9 hours.