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