Two pipes A and B can fill a tank in 20 and 16 hours respectively. Pipe B alone is kept open for 1/4 of time and both pipes are kept open for remaining time. In how many hours, the tank will be full?

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?