60, 61, 62, 63,... 97, 98, 99, 100.

To find the denominator of the probability:

The mistake most people would make here is to assume that there
are 100-60 or 40 possible grades, when there are really 41 possible
grades.  The reason it's 41 and not 40 is this:

1.  There are 100 numbers from 1 to 100
2.  We don't count the numbers from 1 to 59, and there are 59 of them.
3.  Therefore the number of numbers from 60 to 100 inclusive is 100-59 or 41.

To find the numerator of the probability

The same kind of mistake most people would make here is to assume that 
there are 100-90 or 10 possible A grades, when there are really 11 
possible A grades.  The reason it's 11 and not 10 is because

1.  There are 100 numbers from 1 to 100
2.  We don't count the numbers from 1 to 89, and there are 89 of them.
3.  Therefore the number of numbers from 90 to 100 inclusive is 100-89 or 11.


So the probability is 11/41.    

Edwin