Some of 50%-intensity red paint is replaced with 20% solution of red paint such that the new paint intensity is 40%. What fraction of the original paint was replaced?	Select:
A) 1/3
B) 1/5
C) 2/3
D) 3/4
E) 4/5
40% is 20%-points above 20% and 10%-points below 50%.   Thus the ratio of 20%-solution to 50%-solution is 1:2.   1/3 of the original paint was replaced.   The answer is 1/3.
<result>A</result>
Some of 50%-intensity red paint is replaced with 20% solution of red paint such that the new paint intensity is 40%. What fraction of the original paint was replaced?