A water tank is one-fifth full. Pipe A can fill a tank in 15 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 from the following answers:	A) 2	B) 4	C) 6	D) 8	E) 10
The combined rate of filling/emptying the tank = 1/15 - 1/6 = -1/10   Since the rate is negative, the tank will be emptied.   A full tank would take 10 minutes to empty.   Since the tank is only one-fifth full, the time is  (1/5) * 10= 
<gadget id="calculator"> (1/5) * 10</gadget>
<output>2</output>
2 minutes   The answer is 2.
<result>A</result>
A water tank is one-fifth full. Pipe A can fill a tank in 15 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?