Some of 50%-intensity red paint is replaced with 25% solution of red paint such that the new paint intensity is 45%. What fraction of the original paint was replaced?  Choose the correct option:
A) 1/30
B) 1/5
C) 2/3
D) 3/4
E) 4/5
45% is 20%-points above 25% and 5%-points below 50%.   Thus the ratio of 25%-solution to 50%-solution is 1:4.   1/5 of the original paint was replaced.   The answer is 1/5.
<result>B</result>
Some of 50%-intensity red paint is replaced with 25% solution of red paint such that the new paint intensity is 45%. What fraction of the original paint was replaced?