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?

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 = $$\frac{60}{20} = 3\,$$​​

Efficiency of P+ Q = $$\frac{60}{12} = 5$$ 

Efficiency of Q = 5 - 3 = 2 

Pipe Q will drain in = $$\frac{60}{2} = 30 \, hours$$ 

Thus the capacity of the tank = $$30 \times 20 = \bf 600 \, litres$$​​