The ratio of milk to water in 80 litres of a mixture is 7 : 3. The water (in litres) to be added to it to make the ratio 2 : 1 is -

$$\eqalign{ & {\text{Quantity of milk}} \cr & = \left( {80 \times \frac{7}{{10}}} \right){\text{litres}} \cr & = {\text{56 litres}}{\text{.}} \cr & {\text{Quantity of water}} \cr & = \left( {80 - 56} \right){\text{litres}} \cr & = {\text{24 litres}}{\text{.}} \cr} $$
Let the quantity of water to be added be x litres
Then,
$$\eqalign{ & = \frac{{56}}{{24 + x}} = \frac{2}{1} \cr & \Rightarrow 2x = 8 \cr & \Rightarrow x = 4 \cr} $$