Two pipes A and B can fill a cistern in 12 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?	Pick.
A) 15 min B) 13 min C) 12 min D) 10 min E) 17 min
1/12 + 1/20 - 1/30   = 1/10   10/1= 
<gadget id="calculator">10/1</gadget>
<output>10</output>
10   Answer:10 min
<result>D</result>
Two pipes A and B can fill a cistern in 12 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?