Question: A tank is filled by two pipes in 9 hours and 18 hours respectively. If both pipes are opened together, how long will it take to fill the tank?
Options:
Correct Answer: 6 hours
Solution:
The combined rate is 1/9 + 1/18 = 2/18 + 1/18 = 3/18 = 1/6. Therefore, it will take 6 hours to fill the tank.