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A swimming pool can be filled by two pipes in 20 minutes and 30 minutes respectively. Both the pipes are opened together in an empty swimming pool. After some time, the first pipe stopped working, and the remaining swimming pool was filled in 18 minutes. After how much time the first pipe stopped working?

A swimming pool can be filled by two pipes in 20 minutes and 30 minutes respectively. Both the pipes are opened together in an empty swimming pool. After some time, the first pipe stopped working, and the remaining swimming pool was filled in 18 minutes. After how much time the first pipe stopped working? Correct Answer 4.8 minutes

Given:

The first pipe can fill the swimming pool in 20 minutes

The second pipe can fill the swimming pool in 30 minutes

Concept:

If a tap can fill a tank in x hours, then the tank filled by the tap in 1 hour is 1/x of the total tank.

Calculation:

Swimming pool filled by first pipe in 1 min = 1/20 units

Swimming pool filled by second pipe in 1 min = 1/30 units

Swimming pool filled by both pipes in 1 min = {(1/20) + (1/30)}

⇒ 1/12

Second pipe is working from start to end without stopping.

So, swimming pool filled by second pipe in 18 minutes = 18 × (1/30)

⇒ 18/30

⇒ 3/5 units

Now, The quantity of swimming pool filled by both pipes = 1 - (3/5)

⇒ 2/5 units 

So, Time taken by both pipes to fill 2/5 units = (2/5)/(1/12)

⇒ (24/5) minutes

⇒ 4.8 minutes

∴ Pipe A stopped working after 4.8 minutes.

Hint

 

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