A pipe can fill a tank in x hours and another can empty it in y hours. They can together fill it in (y > x )

Let the capacity of the tank be $$xy$$ litres.

Let 1st pipe fills it in $$x$$ hrs and 2nd pipe empties it in $$y$$ hrs

Rate at which 1st pipe fills it = $$\frac{xy}{x} = y$$ l/hr

and rate at which 2nd pipe empties it = $$\frac{xy}{y} = -x$$ l/hr ['-' denotes emptying the tank]

=> Rate at which both pipes fill the tank = $$y+(-x) = (y-x)$$ l/hr

=> Time required to fill it together = $$\frac{xy}{y-x}$$ hours