The average of the first $10$ even numbers is:

The first $10$ even numbers are $2,4,6,8,10,12,14,16,18,20$.
The formula for the sum of the first $n$ even numbers is $n(n+1)$.
For $n=10$:

$$\text{Sum}=10\times (10+1)=110$$

Now find the average:

$$\text{Average}=\frac{\text{Sum}}{n}=\frac{110}{10}=11$$

Alternatively, the formula for the average of the first $n$ even numbers is simply $n+1$. Here, $10+1=11$.