Question: A tank can be filled by a pipe in 9 hours and emptied by another pipe in 3 hours. If both pipes are opened together, how long will it take to fill the tank?
Options:
Correct Answer: 2.25 hours
Solution:
The net rate is 1/9 - 1/3 = 1/9 - 3/9 = -2/9. Therefore, the tank will never fill.