A tap can empty a tank in 30 minutes. A second tap can empty

A tap can empty a tank in 30 minutes. A second tap can empty it in 45 minutes. If both the taps operate simultaneously, how much time is needed to empty the tank ?

As per the given in question , we have

Part of tank emptied by both pipes in 1 minute =	1	+	1	=	3 + 2
	30		45		90

Part of tank emptied by both pipes in 1 minute =	5	=	1
	90		18

∴ Required time = 18 minutes