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A pipe P can drain out water from a tank in 20 hours. Another pipe Q can drain 20 litres per hour. If both drains are opened, the tank is emptied in 1
Question

A pipe P can drain out water from a tank in 20 hours. Another pipe Q can drain 20 litres per hour. If both drains are opened, the tank is emptied in 12 hours. What is the capacity of the tank?

A.

600 litres

B.

800 litres

C.

400 litres

D.

650 litres

Correct option is A

Given:
Pipe P drains the tank in 20 hours.
Pipe Q drains 20 liters per hour.
Combined time to empty the tank: 12 hours. 
Formula Used: 
Total work = Efficiency ×\times Time 
Solution: 
Total work = LCM of 20, 12 = 60 unit 
Efficiency of P = 6020=3 \frac{60}{20} = 3\, ​​
Efficiency of P+ Q = 6012=5\frac{60}{12} = 5 
Efficiency of Q = 5 - 3 = 2 
Pipe Q will drain in = 602=30 hours\frac{60}{2} = 30 \, hours 
Thus the capacity of the tank = 30×20=600 litres30 \times 20 = \bf 600 \, litres​​

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