c
$\frac{\theta_{1}-\theta_{2}}{t}=\left[\frac{\theta_{1}+\theta_{2}}{2}-\theta_{0}\right] K$
$\frac{60-50}{10}=\mathrm{K}[55-25]$
$K=\frac{1}{30}$
$\frac{50-\theta_{1}}{10}=\frac{1}{30}\left[\frac{\theta_{1}+50}{2}-25\right]$
$6\left[50-\theta_{1}\right]=\theta_{1}$
$\Rightarrow 7 \theta_{1}=300$
$\Rightarrow \theta_{1}=\frac{300}{7}$