Pipe And Cistern

Pipes A and B can fill a tank in 40 hours and 50 hours respectively and pipe C can empty the full tank in 60 hours. If all the pipes are opened together, how much time (in hours) will be needed to fill the tank?

Three pipes A, B and C can fill an empty tank together in 8 hours. After working at it together for 2 hours B is closed and A and C can fill the remaining part in 9 hours. Pipe B can fill the empty tank in :