Two pipes x and y can fill a tank in 24 and 30 minutes respectively.

Two pipes x and y can fill a tank in 24 and 30 minutes respectively. Both the pipes are opened for 6 minutes, after which Pipe x is turned off. How much more time will it take to fill the tank?

As per the given in question ,
When Pipe x is turned off (after 6 minutes),

Work done by x and y in 6 minutes = 6		1	+	1		=	9
		24		30			20

Remaining work = 1 -	9	=	11
	20		20

which would be done by Pipe y alone.
1 work is done by Pipe y (alone) in 30 minutes

11	work is done by Pipe y (alone) in 30 ×	11	=	33	minutes = 16.5 minutes.
20		20		2