Question 20

A water tank is $$\frac{2}{5}$$ th full. Inlet A can fill the tank in 12 minutes while an outlet pipe B can empty it in 6 minutes. If both the pipes are open, how long will it take to empty or fill the tank completely?


Pipe A can fill in 12 hours.

Pipe B can empty in 6 hours.

Let's assume total work is 60.

Efficiency of A = 5

Efficiency of B = -10

Tank is $$\frac{2}{5}$$ th full. It means that it's 24 parts filled.

If both pipes A and B are opened. Then efficiency will be -5.

So $$\frac{\text{24 parts}}{-5}$$

-4.8 min. (Here negative sign represent for emptying.)

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