thermal heat transfer

Dear Student,
Assuming that the heat flow is uniform through out the system,
$$\begin{gathered} \frac{{\theta }_1-{\theta }_2}{2m}=\frac{{\theta }_1-80}{1/3 m} \\ \frac{60}{2}=3\left({\theta }_1-80\right) \\ 30=3\left({\theta }_1-80\right) \\ 10={\theta }_1-80 \\ {\theta }_1={90}^{o}C \end{gathered}$$

Ans (C)

Regards