A water tank is one-fifth 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?  Options
A) 3
B) 6
C) 9
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 one-fifth full, the time is  (1/5) * 15= 
<gadget id="calculator"> (1/5) * 15</gadget>
<output>3</output>
3 minutes   The answer is 3.
<result>A</result>
A water tank is one-fifth 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?