Pipes A and B can fill a tank in 4 hours and 6 hours respectively, and pipe C can empty the full tank in 12 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 12 units (the LCM of 4, 6 and 12). Per hour, A fills 3 units, B fills 2 units and C empties 1 unit. Net filling = 3 + 2 − 1 = 4 units per hour. Time = 12 ÷ 4 = 3 hours. Adding C as a filling pipe would give 2 hours, which is too fast.