From the 50 liters of milk 5 liters of milk is taken out and after it 5 liters of water is added to ...........

Explanation:

Final or reduced concentration = initial concentration x (1−amount being replaced in each operation /total amount)ⁿ

where n is the number of times the same operation is being repeated.

The "amount being replaced" could be pure or mixture as per the case.

similarly ,"total amount" could also be either pure or mixture.

Here amount being replaced denotes the quantity which is to be withdrawn in each time.

Therefore,    50×(1−5/50)³

= 36.45 L

Hence, option 3 is the correct answer.