Two pipes A and B can fill a cistern in 15 and 20 minutes respectively, and a third pipe C can empty it in 30 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 1/4 min B) 12 min C) 17 1/7 min D) 17 1/8 min E) 17 5/7 min
1/15 + 1/20 - 1/30 = 5/60 = 1/12   12/1= 
<gadget id="calculator">12/1</gadget>
<output>12</output>
12   Answer:12 min
<result>B</result>
Two pipes A and B can fill a cistern in 15 and 20 minutes respectively, and a third pipe C can empty it in 30 minutes. How long will it take to fill the cistern if all the three are opened at the same time?