$\mathrm{T}_{1}(\mathrm{V})^{\frac{7}{5}-1}=\mathrm{T}_{2}(32 \mathrm{V})^{\frac{7}{5}-1}$
$\mathrm{T}_{1}=4 \mathrm{T}_{2}$
$\eta=1-\frac{T_{2}}{4 T_{2}}=0.75$


Reason $R$ : The efficiency of Carnot's engine depends not only on temperature of cold reservoir but it depends on the temperature of hot reservoir too and is given as $\eta=\left(1-\frac{ T _2}{ T _1}\right)$.
In the light of the above statements, choose the correct answer from the options given below