A water tank is two-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 long will it take to empty or fill the tank completely?  Pick one
A) 6 min
B) 4 min
C) 2 min
D) 5 min
E) 8 min
Clearly, pipe B is faster than pipe A and so, the tank will be emptied.   Part to be emptied = 2/5   Part emptied by (A + B) in 1 minute = (1/6 - 1/10) = 1/15   1/15 : 2/5 :: 1 : x   x = (2/5 * 1 * 15)= 
<gadget id="calculator">(2/5 * 1 * 15)</gadget>
<output>6</output>
6 min.   So, the tank will be emptied in 6 min.   6 minnswer:6 min
<result>A</result>
A water tank is two-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 long will it take to empty or fill the tank completely?