A water tank is three-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 long will it take to empty or fill the tank completely?
Choose the correct choice from the following options
A) 6 min  B) 8 min  C) 7 min  D) 9 min  E) 1 min
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 three-fifths full, the time is  (3/5) * 15= 
<gadget id="calculator"> (3/5) * 15</gadget>
<output>9</output>
9 minutes   The answer is 9 min.
<result>D</result>
A water tank is three-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 long will it take to empty or fill the tank completely?