Pipe D can fill 40% of a tank in 4 hours, and Pipe E can fill 30% of the tank in 6 hours. Pipe F is an outlet pipe and can empty the completely filled tank in 20 hours. If the tank is already 30% filled, find the time taken by all three pipes to fill the remaining part of the tank.

ATQ, Use the unit method for quick calculation. Take total tank capacity = 60 units. Pipe D fills 40% of the tank in 4 hours. 40% of 60 = 24 units. So, efficiency of Pipe D = 24/4 = 6 units per hour. Pipe E fills 30% of the tank in 6 hours. 30% of 60 = 18 units. So, efficiency of Pipe E = 18/6 = 3 units per hour. Pipe F empties the full tank in 20 hours. So, efficiency of Pipe F = 60/20 = 3 units per hour. Since Pipe F is an outlet pipe, its work will be subtracted. Net efficiency of all three pipes = 6 + 3 − 3 = 6 units per hour. The tank is already 30% filled. So, remaining tank = 70%. 70% of 60 = 42 units. Time taken to fill the remaining tank = 42/6 = 7 hours. Therefore, all three pipes together will fill the remaining tank in 7 hours.