(d) From $\eta = 1 - \frac{T_{2}}{T_{1}}\ \Rightarrow \ \frac{T_{2}}{T_{1}} = 1 - \eta = 1 - \frac{400}{100} = \frac{3}{5}$
$\therefore\ \ T_{2} = \frac{3}{5}T_{1} = \frac{3}{5} \times 500 = 300\ K$
Again $\frac{T_{2}}{T_{1}^{'}} = 1 - \eta$ or $\frac{300}{T_{1}^{'}} = 1 - \frac{50}{100} = \frac{1}{2}$
or $T_{1} = 600\ K$