A tap can fill a cistern in 8 hours and another can empty it in 16 hours. If both are opened together, the time (in hours) to fill the tank is:
Answer: C
The filling tap's rate is 1/8 tank/hour. The emptying tap's rate is -1/16 tank/hour. The net rate is \(1/8 - 1/16 = (2-1)/16 = 1/16\) tank/hour. So, the tank will be filled in 16 hours.