Arithmetic Aptitude :: Pipes & Cistern
10. Two pipes A and B can fill a cistern in 20 and 30 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?
Cistern part filled by pipe A in 1 min = 1/20
and by pipe B in 1 min = 1/30
Pipe C empty cistern in 1 min = 1/40 part.
When all pipes are open Cistern part will be filed = (1/20+1/30-1/40) = 7/120
=> Time required to fill the cistern = 120/7 = 17 1/7 min.