From a container having pure milk, 20% is replaced by water and the process is repeated thrice. At the end of the third operation, the milk is :

Answer: Option C

Let total quantity of original milk = 1000 gm
Milk after first operation = 80% of 1000 = 800 gm
Milk after second operation = 80% of 800 = 640 gm
Milk after third operation = 80% of 640 = 512 gm
∴ Strength of final mixture = 51.2%