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 -

Answer: Option A

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