Pipe A can fill a tank in 9 hours and pipe B can empty the full tank in 18 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 1/9 of the tank and B empties 1/18. The net filling per hour is 1/9 − 1/18 = 1/18. So the tank fills in 18 hours. Shortcut: ab ÷ (b − a) = 162 ÷ 9 = 18. Adding the rates would treat B as a filling pipe, which is wrong.