(b) $\eta = 1 - \frac{T_{2}}{T_{1}} \Rightarrow \frac{1}{2} = 1 - \frac{500}{T_{1}} \Rightarrow \frac{500}{T_{1}} = \frac{1}{2}$ …(i)
$\frac{60}{100} = 1 - \frac{T_{2}^{'}}{T_{1}} \Rightarrow \frac{T_{2}^{'}}{T_{1}} = \frac{2}{5}$ …(ii)
Dividing equation (i) by (ii)
$\frac{500}{T_{2}'} = \frac{5}{4} \Rightarrow T_{2}^{'} = 400K$