SOLUTION: One pipe can fill a tank in 45 mins. and another pipe can fill it in 30 mins. If these two pipes are open and a third pipe is draining water from the tank, it takes 27 mins. to fil

From the condition,


    the filling rate of the 1-st pipe is    of the tank volume per minute.


    the filling rate of the 2-nd pipe is    of the tank volume per minute.


Let X be the draining rate of the 3-rd pipe.


Then the balance equation is


     +  - X = ,


so  X =  +  -  =  =  = .


It means that the 3-rd tube will empty full tank in 54 minutes.