| Class: | $10 - 20$ | $20 - 30$ | $30 - 40$ |
| Frequency: | 2 | $x$ | 2 |
If the variance of the following frequency distribution:
\begin{center}
\begin{tabular}{ l l c c }
Class: & $10 - 20$ & $20 - 30$ & $30 - 40$ \\
Frequency: & 2 & $x$ & 2 \\
\end{tabular}
\end{center}
is 50, then $x$ is equal to $\_\_\_\_$