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

From the 50 liters of milk, 5 liters of milk is taken out and after it 5 liters of water is added to the rest amount of milk. Again 5 liters of milk and water is drawn out and it was replaced by 5 liters of water. If this process is continued  similarly for the third time, the amount of milk left after the third replacement:

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Here amount being replaced denotes the quantity which is to be withdrawn in each time.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

By: Amit Kumar