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

Answer: Option A

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