A water tank is one-fourth full. Pipe A can fill a tank in 10 minutes and pipe B can empty it in 8 minutes. If both the pipes are open, how long will it take to empty or fill the tank completely?
Pick one.
A) 4 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 = 1/4   Part emptied by (A + B) in 1 minute = (1/8 - 1/10) = 1/40   1/40 : 1/4 :: 1 : x   x = (1/4 * 1 * 40)= 
<gadget id="calculator">(1/4 * 1 * 40)</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 one-fourth full. Pipe A can fill a tank in 10 minutes and pipe B can empty it in 8 minutes. If both the pipes are open, how long will it take to empty or fill the tank completely?