Pipes A and B can fill a tank in 10 hours and 15 hours respectively, and pipe C can empty the full tank in 30 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 30 units (the LCM of 10, 15 and 30). 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 = 30 ÷ 4 = 7.5 hours. Adding C as a filling pipe would give 5 hours, which is too fast.