Two pipes A and B can fill a cistern in 12 and 24 minutes respectively, and a third pipe C can empty it in 36 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) 10 2/7 min	B) 10 1/8 min	C) 11 1/7 min	D) 12 8/7 min	E) 11 3/7 min
1/12 + 1/24 - 1/36   = 7/72   72/7 = 10 2/7   10 2/7 minnswer:10 2/7 min
<result>A</result>
Two pipes A and B can fill a cistern in 12 and 24 minutes respectively, and a third pipe C can empty it in 36 minutes. How long will it take to fill the cistern if all the three are opened at the same time?