Two pipes A and B can fill a cistern in 20 and 40 minutes respectively, and a third pipe C can empty it in 80 minutes. How long will it take to fill the cistern if all the three are opened at the same time?
Select.
A) 19 1/7 min
B) 15 1/7 min
C) 17 1/7 min
D) 7 1/7 min
E) 16 min
1/20 + 1/40 - 1/80 = 5/80   80/5= 
<gadget id="calculator">80/5</gadget>
<output>16</output>
16   ANSW16 minR:16 min
<result>E</result>
Two pipes A and B can fill a cistern in 20 and 40 minutes respectively, and a third pipe C can empty it in 80 minutes. How long will it take to fill the cistern if all the three are opened at the same time?