alphonsa wrote:

Pipe A and Pipe B fill water into a tank of capacity 1000 litres at the rate of 100 litres/minute and 25 litres/minute respectively. Pipe C drains water from the same tank at the rate of 50litres/minute. Pipe A is kept open for a minute and then closed, then pipe B is kept open for a minute and then closed, followed by a drain pipe C for a minute and then closed. This process is repeated until the tank is full. How long will it take to fill the tank.

A) 39 mins

B) 40 mins

C) 39 mins and 20 secs

D) 37 mins

E) 41 mins

At time 0 minutes, water in the tank = 0 liters

At time 1 minute, water in the tank = 100 liters (0 from before + 100 from pipe A)

At time 2 minutes, water in the tank = 125 liters (100 from before + 25 from pipe B)

At time 3 minutes, water in the tank = 75 liters (125 from before - 50 from pipe C)

If we take these steps as one single step.

At time 6 minutes, water in the tank = 150 liters (75 from before + 75 from the new round)

Now, we can see that after 6*6 = 36 minutes, water in the tank = 150 * 6= 900 liters

Now, since we start the process over again, At time 37 minutes, water in the tank = 1000 liters (900 from before + 100 from pipe A)

Hence D

Hope that makes it clear

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