done in vb.net, want this to be basic, want minor mistakes to see if i can fix them myself

Design and develop a project to assign a letter grade (A >= 90, B = [80..89], C = [70..79], D = [60..69], and F = [0..59]) to a student's Assignment based on a test score and other criteria.

Use a NumericUpDown control to allow the instructor's assistant to enter a test score between 0 and 100.
Use a group of RadioButton controls to determine the grade on Extra Credit Project #1: None (0 points), Average (3 point bonus), Above average (5 point bonus).
Use a second group of RadioButton controls to select whether a student handed in the assignment on time, with the default being On Time. The other choices are 1 to 2 days late (5 point penalty) and more than 2 days late (automatic score of 0).
Display the final total grade in a read-only TextBox control. Use global variables to keep track of which RadioButton control is checked before determining the final grade.
Deliverable Detail

The final grade must be a letter grade (A, B, C, D, or F).  
Declare global variables by placing the Dim statement outside all methods.
gdbExtraCredit: to remember how much extra-credit should be given.
For each radio button, code a sub that handles the CheckedChanged event.
For example:
Private Sub rad1_2DayLate_CheckedChanged (…)
gdbLate = -5
End Sub