Your problem is :
If (Average >= 70) Then
Console.WriteLine("A")
ElseIf (Average >= 60 - 69) Then
Console.WriteLine("B")
ElseIf (Average > 50 - 59) Then
Console.WriteLine("C")
ElseIf (Average >= 40 - 49) Then
Console.WriteLine("D")
ElseIf (Average <= 39) Then
Console.WriteLine("FAIL")
End If
So your code is like :
If (Average >= 70) Then
Console.WriteLine("A")
ElseIf (Average >= 9) Then
Console.WriteLine("B")
ElseIf (Average > 9) Then
Console.WriteLine("C")
ElseIf (Average >= 9) Then
Console.WriteLine("D")
ElseIf (Average <= 39) Then
Console.WriteLine("FAIL")
End If
The correct way to code this is to take advantage of the fact that when you reach a given condition, all previous conditions have failed
If (Average >= 70) Then
Console.WriteLine("A")
ElseIf (Average >= 60) Then
Console.WriteLine("B")
ElseIf (Average > 50) Then
Console.WriteLine("C")
ElseIf (Average >= 40) Then
Console.WriteLine("D")
Else
Console.WriteLine("FAIL")
End If