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

Explanation

The filling rate of the first faucet is 1/4 of the tank per hour, and the draining rate of the second faucet is 1/9 of the tank per hour. The net filling rate when both are open is (1/4) - (1/9) = 5/36 of the tank per hour. Therefore, the tank will be completely filled in 36/5 hours, which equals 7.2 hours.