What is the average of all natural numbers from 21 to 39?

$$\eqalign{ & 21,\,22,\,23\,........\,39 \cr & n = \left( {39 - 21} \right) + 1 \cr & n = 18 + 1 \cr & n = 19 \cr & {S_{19}} = \frac{n}{2}\left( {{\text{first}} + {\text{last}}} \right) \cr & = \frac{{19}}{2}\left( {21 + 39} \right) \cr & = \frac{{19}}{2} \times 60 \cr & = 19 \times 30 \cr & {\text{Average}} = \frac{{{\text{sum}}}}{n} \cr & = \frac{{19 \times 30}}{{19}} \cr & = 30 \cr} $$