Pipes A and B can fill a tank in 9 hours and 12 hours respectively, and pipe C can empty the full tank in 18 hours. If all three pipes are opened together on an empty tank, how long will it take to fill?

💡 Solution & Explanation

Take the tank as 36 units (the LCM of 9, 12 and 18). Per hour, A fills 4 units, B fills 3 units and C empties 2 units. Net filling = 4 + 3 − 2 = 5 units per hour. Time = 36 ÷ 5 = 7.2 hours. Adding C as a filling pipe would give 4 hours, which is too fast.