A portion of the 85% solution of chemicals was replaced with an equal amount of 30% solution of chemicals. As a result, 40% solution of chemicals resulted. What part of the original solution was replaced?
Pick:
A) 5 / 11 B) 6 / 11 C) 7 / 11 D) 9 / 11 E) 13 / 6

Rationale: "this is a weighted average question . say x % of the solution was replaced - - > equate the amount of chemicals : 0.85 ( 1 - x ) + 0.3 * x = 0.4 - - > x = 9 / 11 . answer : d ."
Answer: D