if else Grading scheme

Write a program to calculate the grade of a student from his marks from the following scheme:

90-100 = Ex   

80-90 = A   

70-80 = B   

60-70 = C   

50-60 = D   

<50 = F

In the above problem, if we look at the grading scheme, 60, 70, 80, 90 are in two grade schemes. is this question correct this way? If yes, then, for eg. 90 will be A or Ex.

OR,

the grading scheme should be:

91-100 = Ex   

81-90 = A   

71-80 = B   

61-70 = C   

50-60 = D   

<50 = F


Please suggest.