A mixture contains milk and water in the ratio 5 : 1. On adding 5 litres of water, the ratio of milk to water becomes 5 : 2. Find the quantity of milk in the original mixture.

Given,

Milk : Water = 5 : 1

Let Milk = 5x and Water = x

After adding 5 litres of water, the ratio of milk to water becomes 5 : 2.

$\Rightarrow \dfrac{5x}{x + 5} = \dfrac{5}{2} \\[1em] \Rightarrow 2(5x) = 5(x + 5) \\[1em] \Rightarrow 10x = 5x + 25 \\[1em] \Rightarrow 10x - 5x = 25 \\[1em] \Rightarrow 5x = 25 \\[1em] \Rightarrow x = 5.$

Milk in the original mixture = 5x = 5 × 5 = 25 litres.

Hence, the quantity of milk in the original mixture = 25 litres.