The average of first $10$ natural numbers is

The sum of the first $n$ natural numbers is given by the formula:
$$\text{Sum}=\frac{n(n+1)}{2}$$

For $n=10$:

$$\text{Sum}=\frac{10\times 11}{2}=55$$

The average is total sum divided by $n$:

$$\text{Average}=\frac{\text{Sum}}{n}=\frac{55}{10}=5.5$$