How many numbers are in 2.5, 5.5, 8.5, 11.5,...,80.5,83.5?

I think you must want the number of terms in this series.........we can find this as follows :

a_n  = a₁ + d(n - 1)      where a_n is the last term  = 83.5, a₁ is the first term = 2.5, d is the common difference between terms = 3,   and n is the number of terms........so we have

83.5  = 2.5 + 3(n - 1)      subtract 2.5 from each side

81   = 3(n - 1)        divide both sides by 3

27 = n  -  1        add 1 to both sides

28 = n  = the number of terms