40 litres of a mixture of milk and water contains 10% of water, the water to be added, to make the water content 20% in the new mixture. Find how many litres water will be added ?

Answer: Option D

Water content in 40 litres of mixture.
$$\begin{array}{rl}& =40\times \frac{10}{100}\\ & =4\text{ litres}\end{array}$$
∴ Milk in the mixture.
$$\begin{array}{rl}& \text{ = 40}-4\\ & =36\text{ litres}\end{array}$$
Let x litres of water is mixed
$$\begin{array}{rl}& \Rightarrow \frac{4+x}{40+x}=\frac{20}{100}\\ & \Rightarrow x=5\text{ litres}\end{array}$$