Pipe A can fill a tank in 6 hours and pipe B can empty the full tank in 12 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 1/6 of the tank and B empties 1/12. The net filling per hour is 1/6 − 1/12 = 1/12. So the tank fills in 12 hours. Shortcut: ab ÷ (b − a) = 72 ÷ 6 = 12. Adding the rates would treat B as a filling pipe, which is wrong.