Two pipes A and B can fill a cistern in 8 and 12 minutes respectively, and a third pipe C can empty it in 16 minutes. How long will it take to fill the cistern if all the three are opened at the same time? Choose the correct option
A) 17 1/9 min  B) 5 1/8 min  C) 6 1/7 min  D) 6 6/7 min  E) 17 3/7 min
1/8 + 12 - 1/16   = 7/48   48/7 = 6 6/7   Answer:6 6/7 min
<result>D</result>
Two pipes A and B can fill a cistern in 8 and 12 minutes respectively, and a third pipe C can empty it in 16 minutes. How long will it take to fill the cistern if all the three are opened at the same time?