$\left. \begin{matrix}PV=nRT  \\n=\frac{m}{M}\begin{matrix}{} & {}  \\\end{matrix}  \\\rho =\frac{m}{V}\begin{matrix}{} & {}  \\\end{matrix}  \\\end{matrix} \right\}\Rightarrow P.\frac{m}{\rho }=\frac{m}{M}RT\Rightarrow \rho =\frac{PM}{RT}$ ${{\rho }_{1}}={{\rho }_{2}}\Rightarrow \frac{{{P}_{1}}{{M}_{1}}}{{{T}_{1}}}=\frac{{{P}_{2}}{{M}_{2}}}{{{T}_{2}}}\Rightarrow \frac{4\times 28}{(273+147)}=\frac{3\times 32}{{{T}_{2}}}\Rightarrow {{T}_{2}}=\frac{3\times 32\times 420}{4\times 28}=360K\Rightarrow \theta =360-273=87{}^\circ C$