Pipe M can fill 10% of a tank in 2 hours, and Pipe N can fill 35% of the same tank in 7 hours. Pipe O is an outlet pipe and can empty the completely filled tank in 30 hours. If the tank is already 20% filled, find the time taken by all three pipes together to fill the remaining tank.

ATQ, Use the unit method for quick calculation. Take total tank capacity = 60 units. Pipe M fills 10% of the tank in 2 hours. 10% of 60 = 6 units. So, efficiency of Pipe M = 6/2 = 3 units per hour. Pipe N fills 35% of the tank in 7 hours. 35% of 60 = 21 units. So, efficiency of Pipe N = 21/7 = 3 units per hour. Pipe O empties the full tank in 30 hours. So, efficiency of Pipe O = 60/30 = 2 units per hour. Since Pipe O is an outlet pipe, its work will be subtracted. Net efficiency of all three pipes = 3 + 3 − 2 = 4 units per hour. The tank is already 20% filled. So, remaining tank = 80%. 80% of 60 = 48 units. Time taken to fill the remaining tank = 48/4 = 12 hours. Therefore, all three pipes together will fill the remaining tank in 12 hours.