173. According to the figure below, two metal plates with thermal conductivities $k_1 = 400\,\dfrac{\text{W}}{\text{m.k}}$ and $k_2 = 800\,\dfrac{\text{W}}{\text{m.k}}$ and equal thickness are glued together. The temperatures of the outer surfaces of the plates are $\theta_1 = 0\,^\circ\text{C}$ and $\theta_2 = 90\,^\circ\text{C}$. In steady-state conditions, the temperature at the junction of the two plates is how many degrees Celsius? [Figure: Two rectangular plates placed side by side, labeled with $k_1$, $k_2$, $\theta_1$, and $\theta_2$]
[(1)] $10$
[(2)] $15$
[(3)] $25$
[(4)] $30$
\textbf{173.} According to the figure below, two metal plates with thermal conductivities $k_1 = 400\,\dfrac{\text{W}}{\text{m.k}}$ and $k_2 = 800\,\dfrac{\text{W}}{\text{m.k}}$ and equal thickness are glued together. The temperatures of the outer surfaces of the plates are $\theta_1 = 0\,^\circ\text{C}$ and $\theta_2 = 90\,^\circ\text{C}$. In steady-state conditions, the temperature at the junction of the two plates is how many degrees Celsius?
\textit{[Figure: Two rectangular plates placed side by side, labeled with $k_1$, $k_2$, $\theta_1$, and $\theta_2$]}
\begin{itemize}
\item[(1)] $10$
\item[(2)] $15$
\item[(3)] $25$
\item[(4)] $30$
\end{itemize}
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