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

Rationale: "let total paint = 1 let amount replaced = x 50 ( 1 - x ) + 25 x = 30 x = 4 / 5 answer : e"
Answer: E