How many terms are there in the AP 4.5, 6, 7.5, ..., 30?

a = 4.5, d = 1.5, last term 30. From 30 = 4.5 + (n - 1)1.5 we get 25.5 = 1.5(n - 1), so n - 1 = 17 and n = 18. Reporting 17 stops at the number of steps rather than the number of terms, and 20 comes from dividing 30 by 1.5 and ignoring the first term.