(1) According to the histogram of frequency distribution, the score of the students with the largest number is between 90- 100, which is about 90+1002 = 95;

(2) The annual population is 50.05= 100,

The frequency of passing (score greater than or equal to 90 points) is 1-0.05-0.20=0.75.

The qualified number is 100× 0.75 = 75;

(3) The number of students in each class is arithmetic progression, and the class with the least number of students is 22.

Let the number of students drawn from other classes be A, B and C,

∴3b= 100-22，

∴b=26；

∴a=22+262=24，

c = 26+2 = 28；

The number of students in each class is 22, 24, 26 and 28.