there are two inlets and one outlet to a cistern . one of the inlets takes 2 hours to fill up the cistern and the other inlet takes twice as much time to fill up the same cistern . both of the inlets are turned on at 9 : 00 am with the cistern completely empty , and at 10 : 00 am , the outlet is turned on and it takes 1 more hour to fill the cistern completely . how much time does the outlet working alone takes to empty the cistern when the cistern is full ?