There are certain number of taps fitted to a cistern in which 6 are inlet taps and rest are outlet taps. Each inlet tap can fill the cistern in 12 hours and each outlet tap can empty the cistern in 15 hours and the cistern gets full in 30/7 hours, if all the taps are open. Find the time in which the all the outlet taps (Inlet taps closed) can empty the completely filled tank.