A portion of the 80% solution of chemicals was replaced with an equal amount of 20% solution of chemicals. As a result, 50% solution of chemicals resulted. What part of the original solution was replaced?
Options:
A) 1 / 3 B) 1 / 2 C) 1 / 4 D) 1 / 5 E) 1 / 6

Rationale: "this is a weighted average question . say x % of the solution was replaced - - > equate the amount of chemicals : 0.8 ( 1 - x ) + 0.2 * x = 0.5 - - > x = 1 / 2 . answer : b ."
Answer: B