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 correct choice from the following:
A) 1/30
B) 1/5
C) 2/5
D) 3/4
E) 4/5
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 2/5.
<result>C</result>
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?