A portion of the 85% solution of chemicals was replaced with

A portion of the 85% 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?

A. 5/13
B. 6/13
C. 9/13
D. 13/9
C. 13/6

20 sec solution: : if half of solution A was removed the avg of 85% and 20% would be 52,5%. Since the new solution is 40%, which is closer to 20%, it means that more than half of solution B was used. We then have to find the answer that says that more than half of A was replaced.

Answers A and B say that less than half of solution A was removed

Answer D and E are impossible, we can't remove more than what we have.

Answer C says that more than half was removed = correct.