At a football stadium, 20% of the fans in attendance were teenagers. If there were 40 teenagers at the football stadium, what was the total number of people at the stadium?

Let the total number of people at the stadium be represented by \( x \).

According to the problem, 20% of the fans in attendance are teenagers. This can be expressed mathematically as:

\[ 0.2x = 40 \]

To find \( x \), we need to solve the equation. We do this by dividing both sides by 0.2:

\[ x = \frac{40}{0.2} \]

Calculating the right side:

\[ x = 40 \div 0.2 = 40 \times 5 = 200 \]

Therefore, the total number of people at the stadium is

\[ \boxed{200} \]