If 10 gallons of grape juice are added to 40 gallons of a mixture, which contains 20 percent grape juice then what percent of the resulting mixture is grape juice? Choose the correct choice from the following choices.
A) 14 %
B) 25 %
C) 28 %
D) 36 %
E) 50 %

Rationale: official solution : if we start with 40 gallons of a mixture that is 10 % grape juice , then we have : 40 × 0.20 = 8 gallons of grape juice . 40 × 0.80 = 32 gallons of other components . if we add 10 gallons of grape juice , we will end up with 18 gallons of grape juice and 32 gallons of other components , and we will have a total of 50 gallons of the mixture . so 18 / 50 of the new mixture is grape juice . now we convert this to a percent : percent grape juice = 18 / 50 = 36 / 100 = 36 % . the correct answer is choice ( d )
Answer: D