A cistern can be filled by a tap in 4 hours and emptied by an outlet pipe in 6 hours. How long will it take to fill the cistern if both the tap and the pipe are opened together?
Given,
Inlet tap can fill a cistern in = 4 hours
Outlet tap can emptied it in = 6 hours
∵ Work done by inlet tap in 1 hour = 
∵ Work done by outlet pipe in 1 hour = 
∴ Work done by both pipe in 1 hour =
Hence, both the tap can filled it in = 
28