A cistern is filled by pipe A in 16 hours and the full cistern can be leaked out by an exhaust pipe B in 20 hours. If both the pipes are opened, in what time the cistern is full?  Pick one:
A) 50hrs	B) 60hrs	C) 70hrs	D) 80hrs	E) 90hrs
time taken to full the cistern=(1/16-1/20)hrs   =1/80   =80hrs   ANSWER:80hrs
<result>D</result>
A cistern is filled by pipe A in 16 hours and the full cistern can be leaked out by an exhaust pipe B in 20 hours. If both the pipes are opened, in what time the cistern is full?