Pipes A and B can fill a tank in 12 hours and 15 hours respectively, and pipe C can empty the full tank in 20 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 60 units (the LCM of 12, 15 and 20). Per hour, A fills 5 units, B fills 4 units and C empties 3 units. Net filling = 5 + 4 − 3 = 6 units per hour. Time = 60 ÷ 6 = 10 hours. Adding C as a filling pipe would give 5 hours, which is too fast.