A water tank is two-third 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
A) 6 min. to empty B) 6 min. to full C) 10 min. to empty D) 9 min. to full E) 8 min. to full
Clearly, pipe B is faster than pipe A and so, the tank will be emptied.   Part to be emptied = 2/3   Part emptied by (A + B) in 1 minute = (1/6 - 1/10) = 1/15   1/15 : 2/3 :: 1 : x   x = (2/3 * 1 * 15)= 
<gadget id="calculator">(2/3 * 1 * 15)</gadget>
<output>10</output>
10 min.   So, the tank will be emptied in 10 min.   ANSWER:10 min. to empty
<result>C</result>
A water tank is two-third 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?