7. (10 points) The average age of everyone in the classroom is 11 years old. If we exclude a 30-year-old teacher, the average age of the rest is 10 years old. How many people are in the classroom?
【Answer】Solution: Let there be $x$ people in the classroom,
$$
\begin{aligned}
11 x-10 \times(x-1) & =30 \\
11 x-10 x+10 & =30 \\
x+10 & =30 \\
x & =20
\end{aligned}
$$

Answer: There are 20 people in the classroom.