Question 32

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?

Solution

Let say, P drains out x ltrs in 20 hours.

So, in 1 hour P drains out $$\frac{x}{20}\ ltrs.$$

or, in 12 hours P drains out $$\frac{12x}{20}\ ltrs=\frac{3x}{5}\ ltrs.$$

In 12 hours Q drains out =$$12\times20=240\ ltrs.$$

So, $$\frac{3x}{5}=240\ .$$

or,$$x=240\times\frac{5}{3}=400.$$

C is correct choice.

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