b
$\mathrm{PV}=\frac{4}{\mathrm{M}} \mathrm{RT}$$...(i)$
$\mathrm{PV}=\frac{(4-0.8)}{\mathrm{M}} \mathrm{R}(\mathrm{T}+50)$
$=\frac{3.2 \mathrm{R}}{\mathrm{M}}(\mathrm{T}+50)$$...(ii)$
By $(i)$ and $(ii)$ we get,
$\frac{4}{M} R T=\frac{3.2 R}{M}(T+50)$
$\Rightarrow 4 T=3.2 T+3.2 \times 50$
$\Rightarrow \mathrm{T}=200 \mathrm{K}$