A portion of the 50% solution of chemicals was replaced with an equal amount of 60% solution of chemicals. As a result, 55% solution of chemicals resulted. What part of the original solution was replaced?  Choose the correct answer.
A) 3 / 4  B) 1 / 4  C) 3 / 4  D) 2 / 5  E) 1 / 2

Rationale: "this is a weighted average question . say x % of the solution was replaced - - > equate the amount of chemicals : 0.5 ( 1 - x ) + 0.6 * x = 0.55 - - > x = 1 / 2 . answer : e ."
Answer: E