A mixture of water and milk contains 80% milk. In 50 litres of such a mixture, how many litres of water is required to increase the percentage of water to 25%?

3.33

Quantity of milk in the mixture = 50 x \(\frac{80}{100}\) = 40

Let the quantity of water to be added be x,

Then the quantity of milk reduces to 75%

\(\frac{10 + x}{40}\) = \(\frac{25}{75}\) 

3 x (10 + x) = 40

3x = 10

x = 3.33