1. First of all, the total score of the primary four subjects of politics, history, geography, and biology is 100 points.

2. Next, assign points between 80-100 points, the grade is A, and the converted score is 20 points.

3. Then, between 70 and 79 points, the grade is B, and the converted score is 16 points. Between 60 and 69 points, the grade is C, and the converted score is 12 points.

4. The final score is between 0-59, the grade is D, and the converted score is 8 points.