A heat engine operates between a cold reservoir at temperature T 2 = 400   K and a hot reservoir at…

Recall the formula of efficiency of heat engine in terms of work done and heat supplied to the source, $\eta =\frac{W}{Q_{\mathrm{H}}}=1-\frac{T_{\mathrm{L}}}{T_{\mathrm{H}}}$, here $T_L \& T_H$, is the temperature of the sink and source$\Rightarrow \frac{60}{300}=1-\frac{400}{T_H}$$\Rightarrow T_{\mathrm{H}}=500 \mathrm{K}$.