An inlet pipe fills a tank in 6 hours, while a drain empties it in 9 hours. If both are open, how long will it take to fill the empty tank?
Correct answer: D. 18 hours
The net rate is 1/6 − 1/9 = 3/18 − 2/18 = 1/18 of the tank per hour, so it takes 18 hours. The drain works against the inlet, so its rate is subtracted.
hard