Two pipes A and B can fill a cistern in 9 and 18 minutes respectively, and a third pipe C can empty it in 24 minutes. How long will it take to fill the cistern if all the three are opened at the same time?
Select the correct option:
A) 6 min
B) 7 min
C) 8 min
D) 10 min
E) 17 min
1/9 + 1/18 - 1/24   = 1/8   8/1= 
<gadget id="calculator">8/1</gadget>
<output>8</output>
8   Answer:8 min
<result>C</result>
Two pipes A and B can fill a cistern in 9 and 18 minutes respectively, and a third pipe C can empty it in 24 minutes. How long will it take to fill the cistern if all the three are opened at the same time?