20 litres of a mixture contains milk and water in the ratio 3 : 1. Then the amount of milk to be added to the mixture so as to have milk and water in ratio 4 : 1 is-

Answer: Option C

In 20 litres of mixture
Quantity of milk
$$\Rightarrow \frac{3}{4}\times 20=15\text{ litres}$$
Quantity of water
$$\Rightarrow \frac{1}{4}\times 20=5\text{ litres}$$
Let the quantity of milk added be x litres.
According to the question,
$$\begin{array}{rl}& \Rightarrow \frac{15+x}{5}=\frac{4}{1}\\ & \Rightarrow 15+x=4\times 5\\ & \Rightarrow x=20-15\\ & \Rightarrow x=5\text{ litres}\end{array}$$