12. In a math exam of a class, the average score of all students who got excellent grades is 95 points, and the average score of students who did not get excellent grades is 80 points. It is known that the average score of the whole class is no less than 90 points. What is the minimum proportion of students who got excellent grades in the whole class?
12. Solution: To ensure the average score of the entire class reaches 90 points, it is necessary to match 2 excellent students with 1 non-excellent student, meaning the number of excellent students must be at least twice the number of non-excellent students to ensure the average score of the entire class is no less than 90 points. Therefore, the proportion of excellent students in the entire class should be at least $\frac{2}{3}$.