A cistern is filled by pipe A in 10 hours and the full cistern can be leaked out by an exhaust pipe B in 15 hours. If both the pipes are opened, in what time the cistern is full?  Pick:  A) 50hrs  B) 30hrs  C) 70hrs  D) 80hrs  E) 90hrs
time taken to full the cistern=(1/10-1/15)hrs   =1/30   =30hrs   ANSWER:30hrs
<result>B</result>
A cistern is filled by pipe A in 10 hours and the full cistern can be leaked out by an exhaust pipe B in 15 hours. If both the pipes are opened, in what time the cistern is full?