Two pipes A and B can fill a cistern in 20 and 30 minutes respectively, and a third pipe C can empty it in 60 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 answers:
A) 15 min	B) 15 1/7 min	C) 17 1/7 min	D) 7 1/7 min	E) 7 2/7 min
1/20 + 1/30 - 1/60 = 4/60   60/4= 
<gadget id="calculator">60/4</gadget>
<output>15</output>
15   15 minNSWER:15 min
<result>A</result>
Two pipes A and B can fill a cistern in 20 and 30 minutes respectively, and a third pipe C can empty it in 60 minutes. How long will it take to fill the cistern if all the three are opened at the same time?