A milk vendor has 40 litres of a 40% milk solution. How many litres of pure water should be added so that the resulting mixture is a 25% milk solution?

Correct option is C

Given:
Initial volume of the milk solution = 40 litres
Percentage of milk in the initial solution = 40%
Desired percentage of milk in the final solution = 25%
Concept Used:
The amount of milk remains constant, while water is added to dilute the solution.
Solution:
Let the amount of pure water to be added be  x litres.
The amount of milk in the initial solution =$$\frac{40 \times 40}{100}$$​ = 16  litres.
In the final mixture, the total volume becomes 40 + x  litres, and the amount of milk must be 25% of the total volume.

So, the amount of milk in the final solution is 0.25$$\times$$​ (40 + x) .
Setting up the equation:
16 = 0.25$$\times$$​ (40 + x)
16 = 10 + 0.25x
16 - 10 = 0.25x
6 = 0.25x
x = $$\frac{6}{0.25}$$​
x = 24
The vendor should add 24 litres of pure water.

Option (C) is right.