Question

47
Two pipes A and B can fill a cistern in 30 and 20
cistern together but pipe A is closed few minutes and pipe C fills the remaining part of
the cistern in 5 minutes. After how many mintue was pipe A closed
(b) 10 minutes
(c) 11
(a) 9 minutes​

Answer 1

Answer:

In one minute pipe A can fill 1/20

In one minute Pipe B can fill 1/30

In one minute pipe C can empty 1/15

So in one minute 1/20+1/30–1/15 of tank will be full.

3/60+2/60–4/60=1/60 will be full.

So it takes 60 minutes to fill the tank if all the pipes are opened together.

Now the question is if pipe A is opened for a minute and closed and pipe B is opened for a minute and closed and pipe C is opened and closed for a minute successfully

Then it takes three minutes to what will happen normally in one minute if all pipes are open.

The tank will be full by 3*60 =180 minutes. But we need to calculate logically

In 165 minutes it will be full upto 55/60

In 166 minutes it will be 55/60 +1/20=58/60

In 167 minutes it will be 58/60+1/30=60/60.

So the tank will be full by 167 minutes and again we don't have to open the pipe C.

So the tank will be full in 167 minutes and the process of opening and closing has to stop.

hope this is help you☺ friend