A water tank is four-fifths full. Pipe A can fill a tank in 10 minutes and pipe B can empty it in 6 minutes. If both the pipes are open, how many minutes will it take to empty or fill the tank completely?
Choose the correct choice:
A) 6 B) 9 C) 10 D) 12 E) 15
The combined rate of filling/emptying the tank = 1/10 - 1/6 = -1/15   Since the rate is negative, the tank will be emptied.   A full tank would take 15 minutes to empty.   Since the tank is only four-fifths full, the time is  (4/5) * 15= 
<gadget id="calculator"> (4/5) * 15</gadget>
<output>12</output>
12 minutes   The answer is 12.
<result>D</result>
A water tank is four-fifths full. Pipe A can fill a tank in 10 minutes and pipe B can empty it in 6 minutes. If both the pipes are open, how many minutes will it take to empty or fill the tank completely?