A portion of the 90% solution of chemicals was replaced with an equal amount of 20% solution of chemicals. As a result, 40% solution of chemicals resulted. What part of the original solution was replaced?  Choices
A) 5 / 7	B) 6 / 7	C) 9 / 5	D) 13 / 9	E) 13 / 6

Rationale: this is a weighted average question . say x % of the solution was replaced - - > equate the amount of chemicals : 0.90 ( 1 - x ) + 0.2 * x = 0.4 - - > x = 5 / 7 . answer : a .
Answer: A