Some of 50%-intensity red paint is replaced with 20% solution of red paint such that the new paint intensity is 30%. What fraction of the original paint was replaced? Answers:	A) 1/30	B) 1/5	C) 2/3	D) 3/4	E) 4/5
30% is 10%-points above 20% and 20%-points below 50%.   Thus the ratio of 25%-solution to 50%-solution is 2:1.   2/3 of the original paint was replaced.   The answer is 2/3.
<result>C</result>
Some of 50%-intensity red paint is replaced with 20% solution of red paint such that the new paint intensity is 30%. What fraction of the original paint was replaced?