Which term of the AP 6, 6.5, 7, 7.5, ... is equal to 20?

a = 6, d = 0.5. From 20 = 6 + (n - 1)0.5 we get 14 = 0.5(n - 1), so n - 1 = 28 and n = 29. The value 28 is the number of steps needed, not the term number, and 40 comes from dividing 20 by 0.5 while ignoring the first term.