| $x$ | 27 | 29 | 31 | 33 | 35 |
| $f$ | 7 | 15 | 13 | 11 | 9 |
\textbf{142--} In the following frequency table, what is the standard deviation using the quick method?
\begin{center}
\begin{tabular}{|c|ccccc|}
\hline
$x$ & 27 & 29 & 31 & 33 & 35 \\
\hline
$f$ & 7 & 15 & 13 & 11 & 9 \\
\hline
\end{tabular}
\end{center}
(1) $2.6$ \qquad (2) $2.7$ \qquad (3) $2.8$ \qquad (4) $2.9$