Question asks us to find the mean grade received by the left-handed students?

If the mean grade of x students in a particular class is 90
Formula for Mean = Total sum/n or Total sum = mean *n
Given
Number of students (n)=x
Total sum = 90x

1) There are a total of 20 students in the class.
You cant find the left-handed students average with this.... all we can say is 20*90 is the total sum of grades in the class.
Not sufficient

2) There are x/5 right-handed students in the class; together, they receive an average score of 95.
So left handed student should be 4/5.

Sum of grades of right handed students =mean * number of students= 95*x/5 = 14x

Total sum of grades of students in class - total sum of grades of right handed students = total grades of left handed students

90x-14x = 76x = total grades of left handed students

Mean = Total grades of left handed students / total number of students
= 76x/(4x/5) = (76x*5)/4x = 85

Thus Sufficient B