Some of 10%-intensity red paint is replaced with 20% solution of red paint such that the new paint intensity is 15%. What fraction of the original paint was replaced?
Choose one:
A) 2 / 5
B) 2 / 3
C) 1 / 2
D) 1 / 3
E) 1 / 5

Rationale: "let total paint = 1 let amount replaced = x 10 ( 1 - x ) + 20 x = 15 x = 1 / 2 answer : c"
Answer: C