The formula for the efficiency of Carnot engine Working between temperature limits $T_1 \mathrm{and} T_2.$$\left(T_1>T_2\right)$

$\eta =\left(1-\frac{T_2}{T_1}\right)\times 100$

For $\eta =40 \%, T_1=500 \mathrm{K}$

$40=\left(1-\frac{T_2}{500}\right)\times 100$

$T_2=300 \mathrm{K}$

For $\eta =60 \%,T_2=300 \mathrm{K}$ (the source temperature of Carnot
engine for the same sink temp),

$\Rightarrow 60=\left(1-\frac{300}{T_1}\right)\times 100$

$\Rightarrow T_1=750 \mathrm{K}$