A pipe can fill a tank in ‘x’ hours and another pipe

A pipe can fill a tank in ‘x’ hours and another pipe can empty it in‘y’ (y > x) hours. If both the pipes are open, in how many hours will the tank be filled ?

From the question ,

Part of the tank filled in 1 hour =	1
	x

Part of the tank emptied in 1 hour =	1
	y

When both are opened , then

Part of the tank filled in 1 hour =	1	-	1	=	y - x
	x		y		xy

∴ Tank will be filled in	xy	hours
	y - x