I suggest you use the formula for an arithmetic series to calculate that.
The prime numbers that are greater than 80 and less than 100 are 83, 89, and 97.
51-99
24 of them.
99
One: 97
91 93 97
420
49
83 89 97.
Not at all. For example: gcf(101, 102) = 1 gcf(40, 80) = 40
45 of them.
Nineteen. They are: 5101520253035404550556065707580859095