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-

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,
$$\eqalign{ & \Rightarrow \frac{{15 + x}}{5} = \frac{4}{1} \cr & \Rightarrow 15 + x = 4 \times 5 \cr & \Rightarrow x = 20 - 15 \cr & \Rightarrow x = 5{\text{ litres}} \cr} $$