How many litres of water should be added to a 30 litre mixture of milk and water containing milk and water in the ratio of 7 : 3 such that the resultant mixture has 40% water in it?

Answer: Option A

Total quantity of mixture
$$\begin{array}{rl}& =\frac{7}{7+3}\times 30\text{ litres}\\ & =21\text{ litres}\end{array}$$
And quantity of water in the mixture
$$\begin{array}{rl}& =\frac{3}{7+3}\times 30\text{ litres}\\ & =9\text{ litres}\end{array}$$
Let water to be mixed 'a' litre
Then,
$$\begin{array}{rl}& \Rightarrow \left(30+a\right)\times \frac{40}{100}=9+a\\ & \Rightarrow 120+4a=90+10a\\ & \Rightarrow 120-90=10a-4a\\ & \Rightarrow 30=6a\\ & \Rightarrow a=5\end{array}$$
Hence, 5 litres water mixed in the mixture.