Two pipes A and B can fill a cistern in 20 and 10 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 most appropriate option:	A) 19 1/7 min	B) 8 min	C) 17 1/7 min	D) 7 1/7 min	E) 7 2/7 min
1/20 + 1/10 - 1/40 = 5/40   40/5= 
<gadget id="calculator">40/5</gadget>
<output>8</output>
8   ANSWER:8 min
<result>B</result>
Two pipes A and B can fill a cistern in 20 and 10 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?