If 10 gallons of grape juice are added to 30 gallons of a mixture, which contains 10 percent grape juice then what percent of the resulting mixture is grape juice? Choose the correct choice from the following answers: A) 14 % B) 25 % C) 28 % D) 32.5 % E) 50 %

Rationale: "official solution : if we start with 40 gallons of a mixture that is 10 % grape juice , then we have : 30 × 0.10 = 3 gallons of grape juice . 30 × 0.90 = 27 gallons of other components . if we add 10 gallons of grape juice , we will end up with 13 gallons of grape juice and 36 gallons of other components , and we will have a total of 40 gallons of the mixture . so 13 / 40 of the new mixture is grape juice . now we convert this to a percent : percent grape juice = 32,5 % . the correct answer is choice ( d )"
Answer: D