A tank can be filled by two pipes in 20 minutes and 30 minutes

A tank can be filled by two pipes in 20 minutes and 30 minutes respectively. When the tank was empty, the two pipes were opened. After some time, the first pipe was stopped and the tank was filled in 18 minutes. After how much time of the start was the first pipe stopped?

Let the first pipe be closed after t minutes .
According to question ,

∴	t	+	18	= 1
	20		30

⇒	t	= 1 -	18	= 1 -	3	=	2
	20		30		5		5

⇒ t =	2	× 20 = 8 minutes;
	5

Second method to solve this question :
Here, p = 20, q = 30, t = 18

Required time =		p		1 -	t
					q

[∵ first pipe is closed]

Required time =		20		1 -	18
					30

Required time = 20 ×	12	= 8 minutes;
	30