In a mixture 60 litres, the ratio of milk and water 2 : 1. If this ratio is to be 1 : 2, then the quanity of water to be further added is:

Answer: Option D

Quantity of milk
$$=\left(60\times \frac{2}{3}\right)\text{litres}=40\text{litres}$$
Quantity of water in it = (60 - 40) litres = 20 litres
New ratio = 1 : 2
Let quantity of water to be added further be x litres
$$\begin{array}{rl}& \text{Then,}\text{milk : water}=\frac{40}{20+x}\\ & \text{Now},\frac{40}{20+x}=\frac{1}{2}\\ & \Rightarrow 20+x=80\\ & \Rightarrow x=60\\ & \therefore \text{Quantity}\text{of}\text{water}\text{to}\text{be}\text{added = 60}\text{litres}\text{.}\end{array}$$