Some of 50%-intensity red paint is replaced with 25% solution of red paint such that the new paint intensity is 40%. What fraction of the original paint was replaced? Choose the most appropriate option.
A) 1 / 30
B) 1 / 5
C) 2 / 5
D) 3 / 4
E) 4 / 5

Rationale: "40 % is 15 % - points above 25 % and 10 % - points below 50 % . thus the ratio of 25 % - solution to 50 % - solution is 2 : 3 . 2 / 5 of the original paint was replaced . the answer is c ."
Answer: C