Two pipes A and B can separately fill a cistern in 24 minutes and 48 minutes respectively. There is a third pipe in the bottom of the cistern to empty it. If all the three pipes are simultaneously opened, then the cistern is full in 12 minutes. In how much time, the third pipe alone can empty the cistern?
Pick.
A) 90 min B) 48 min C) 54 min D) 120 min E) 130 min
1/12-(1/24+1/48)=-1/48   third pipe can empty in 48 minutes   ANSWER:48 min
<result>B</result>
Two pipes A and B can separately fill a cistern in 24 minutes and 48 minutes respectively. There is a third pipe in the bottom of the cistern to empty it. If all the three pipes are simultaneously opened, then the cistern is full in 12 minutes. In how much time, the third pipe alone can empty the cistern?