Question: If a tank can be filled by a pipe in 6 hours and emptied by another pipe in 9 hours, how long will it take to fill the tank if both pipes are opened together?
Options:
Correct Answer: 4 hours
Solution:
The combined rate is (1/6 - 1/9) = (3-2)/18 = 1/18. Therefore, it will take 18 hours to fill the tank.