| Class : | $0-10$ | $10-20$ | $20-30$ | $30-40$ | $40-50$ |
| Frequency : | 2 | 3 | $x$ | 5 |
If the mean of the frequency distribution
\begin{center}
\begin{tabular}{llllll}
Class : & $0-10$ & $10-20$ & $20-30$ & $30-40$ & $40-50$ \\
Frequency : & 2 & 3 & $x$ & 5 & \\
\end{tabular}
\end{center}
is 28, then its variance is $\_\_\_\_$.