If a pipe can fill a tank in 6 hours and another pipe can empty it in 8 hours, how long will it take to fill the tank if both pipes are opened together?
Answer: B. 12 hours
The filling rate is 1/6 and the emptying rate is 1/8. Together, they fill at a rate of 1/6 - 1/8 = 1/24. Therefore, it will take 24 hours to fill the tank.