A certain quantity of 80% solution is replaced with 25% solution such that the new concentration is 35%. What is the fraction of the solution that was replaced? Select:
A) 1 / 4 B) 1 / 3 C) 1 / 2 D) 2 / 3 E) 9 / 11

Rationale: "let ' s say that the total original mixture a is 100 ml the original mixture a thus has 50 ml of alcohol out of 100 ml of solution you want to replace some of that original mixture a with another mixture b that contains 25 ml of alcohol per 100 ml . thus , the difference between 80 ml and 25 ml is 55 ml per 100 ml of mixture . this means that every time you replace 100 ml of the original mixture a by 100 ml of mixture b , the original alcohol concentration will decrease by 25 % . the question says that the new mixture , let ' s call it c , must be 35 % alcohol , a decrease of only 45 % . therefore , 45 out of 55 is 9 / 11 and e is the answer ."
Answer: E