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 options:
A) 40 % pure	B) 50 % pure	C) 51.2 % pure	D) 58.8 % pure	E) none

Rationale: "solution let total quantity of original milk = 100 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 mixtures = 51.2 % . answer c"
Answer: C