A milkman makes 20% profit by selling milk mixed with water at Rs. 9 per litre, if the cost price of 1 litre pure milk is Rs. 10, then the ratio of milk and water in the said mixture is = ?

Answer: Option A

According to question,
S.P. at 20% profit = Rs. 9/litres
Cost of mixture $$=\frac{9}{120}\times 100=\text{Rs}\text{.}7.5/\text{litre}$$
Now, let the ratio of milk and water in the mixture = x : y
$$\begin{array}{rl}& \Rightarrow \frac{\left(10\times x\right)+\left(0\times y\right)}{x+y}=7.5\\ & \Rightarrow 10x=7.5\left(x+y\right)\\ & \Rightarrow x:y=3:1\end{array}$$