Pipe A can fill a tank in 12 hours and pipe B can empty the full tank in 20 hours. If both pipes are opened together on an empty tank, how long will it take to fill?

💡 Solution & Explanation

In one hour A fills 1/12 of the tank and B empties 1/20. The net filling per hour is 1/12 − 1/20 = 1/30. So the tank fills in 30 hours. Shortcut: ab ÷ (b − a) = 240 ÷ 8 = 30. Adding the rates would treat B as a filling pipe, which is wrong.