In other words, the average score of A, B, C and D is 5 points higher than that of A, B and C..
Therefore, Ding's score is 5×4=20 points higher than the average scores of A, B and C..
The scores of Party A and Party B are 80 points and 82 points respectively, and the full score is 100, so the average score of Party A, Party B and Party C is at most 80 points, otherwise the score of Party D will be greater than 100 points. So the highest score of C is 78, so it is certain that the score of D is higher than that of C.
If the grade of C is X, the grade of D is (80+82+x)÷3+20.
The score of D is higher than that of C: (80+82+x)÷3+20-x=74-2x/3.
So the lower the score of C, the greater the difference between their scores. So when the score of C is 0, their score difference is the largest, 74 points.
Therefore, Ding's score is higher than C's, with the highest score of 74.