Using \(z_{0.005} = 2.576\) and \(s = \sqrt{12.1} \approx 3.4786\) gives a confidence interval
\begin{equation*}
25.3 - 2.576 \cdot \frac{3.4786}{11} \lt \mu \lt 25.3 + 2.576 \cdot \frac{3.4786}{11}
\end{equation*}
or
\begin{equation*}
24.4854 \lt \mu \lt 26.1148.
\end{equation*}