If 20 gallons of grape juice are added to 40 gallons of a mixture, which contains 10 percent grape juice then what percent of the resulting mixture is grape juice?
Choose the correct answer
A) 14 % B) 25 % C) 28 % D) 34 % E) 40 %

Rationale: "official solution : if we start with 40 gallons of a mixture that is 10 % grape juice , then we have : 40 × 0.10 = 4 gallons of grape juice . 40 × 0.90 = 36 gallons of other components . if we add 20 gallons of grape juice , we will end up with 24 gallons of grape juice and 36 gallons of other components , and we will have a total of 60 gallons of the mixture . so 24 / 60 of the new mixture is grape juice . now we convert this to a percent : percent grape juice = 40 / 100 = 40 % . the correct answer is choice ( e )"
Answer: E