A tank has two pipes. The first pipe can fill it in 4 hours

A tank has two pipes. The first pipe can fill it in 4 hours and the second can empty it in 16 hours. If two pipes be opened together at a time, then the tank will be filled in

As per the given in question , we have

Part of tank filled by both the pipes in 1 hour =	1	-	1
	4		16

Part of tank filled by both the pipes in 1 hour =	4 - 1	=	3
	16		16

∴ Required time =	16	= 5	1	hours
	3		3