A tank can be filled by one tap in 4 hours, while another tap can drain it in 9 hours. If both taps are opened at the same time, how long will it take to fill the tank completely?

Explanation

The filling rate of the first tap is 1/4 tank per hour, and the draining rate of the second tap is 1/9 tank per hour. When both taps are open, the net filling rate is (1/4) - (1/9) = 5/36 tank per hour. Therefore, the time taken to fill the tank is the reciprocal of this rate, which is 36/5 = 7.2 hours, or 7 hours and 12 minutes.