Two pipes A and B can fill a cistern in 15 and 30 minutes respectively, and a third pipe C can empty it in 40 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 options
A) 17 1/4 min	B) 28 1/7 min	C) 17 1/7 min	D) 13 3/7 min	E) 17 5/7 min
1/15 + 1/30 - 1/40 = 9/120   120/9 = 13 3/7   Answer:13 3/7 min
<result>D</result>
Two pipes A and B can fill a cistern in 15 and 30 minutes respectively, and a third pipe C can empty it in 40 minutes. How long will it take to fill the cistern if all the three are opened at the same time?