A water tank is three-fifths 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? Pick:  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 three-fifths full, the time is  (3/5) * 10= 
<gadget id="calculator"> (3/5) * 10</gadget>
<output>6</output>
6 minutes   The answer is 6.
<result>C</result>
A water tank is three-fifths 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?