∴a=5÷50=0. 1； b=5÷50=0. 1，d = 1-0. 1-0. 1-0.5 = 0.3，c=50×0.3= 15，

Then c = 50d.

(2) As shown:

(3) According to the meaning of the question, the median of 9 categories (1) is between 5 1 ~ 75; The median value of category 9 (2) is between 76 and 100.

Then the class with a higher score in this math exam is Class 9 (2).

So the answer is: nine (2)