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?  Choose the correct choice from the following choices
A) 45 %
B) 49.6 %
C) 51.2 %
D) 55 %
E) 60.3 %

Rationale: let the total quantity of original milk = 1000 gm milk after 1 st 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 % answer is c
Answer: C