What is the average of all the two-digit whole numbers that are multiples of 9?

The two-digit multiples of 9 run 18, 27, 36, ... , 99 - ten of them, evenly spaced. For an evenly spaced list the average is the midpoint of the two ends: (18 + 99)/2 = 58.5. (The total is 585 and 585/10 = 58.5.) 49.5 is the average of the first ten multiples of 9, that is 9 to 90, and 9 is not a two-digit number. 63 is (27 + 99)/2, which starts the list at 27 and drops 18. 54 is the fifth of the ten numbers, picked as though a list of ten had a single middle number - with an even count there is no middle number, only a midpoint between two.