Two pipes A and B can fill a cistern in 8 and 12 minutes respectively, and a third pipe C can empty it in 15 minutes. How long will it take to fill the cistern if all the three are opened at the same time?
Choose the correct choice from the following choices
A) 17 1/17 min  B) 28 1/7 min  C) 17 1/7 min  D) 17 1/8 min  E) 17 5/7 min
1/8 + 1/12 - 1/15 = 17/120   120/17 = 7 1/17   17 1/17 minnswer:17 1/17 min
<result>A</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 15 minutes. How long will it take to fill the cistern if all the three are opened at the same time?