If a milkman adds 4 l of water to 9 l of milk, then the amount of water he should add to 36 l of milk in order to maintain the same proportion of milk and water will be:

Correct option is A

Given:
Water added to 9 liters of milk = 4 liters.
Milk = 36 liters.
Required: Water to maintain the same proportion.
Formula Used:
Water required = $$\text{Milk volume} \times \frac{\text{Water in initial mixture}}{\text{Milk in initial mixture}}$$​  
Solution:
Initial milk-to-water ratio:
Milk : Water = 9 : 4
Proportion of water in the mixture:
​$$\frac{\text{Water}}{\text{Milk}} = \frac{4}{9}$$​​
Water required for 36 liters of milk:
Water required = $$36 \times \frac{4}{9}$$​= 16liters.