Pipe A can fill a tank in 4 hours and pipe B can empty the full tank in 6 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 1/4 of the tank and B empties 1/6. The net filling per hour is 1/4 − 1/6 = 1/12. So the tank fills in 12 hours. Shortcut: ab ÷ (b − a) = 24 ÷ 2 = 12. Adding the rates would treat B as a filling pipe, which is wrong.