The density of air at 380 K and 722mm of $Hg$ is $1g/c{m}^3$  . If air is cooled to 100 K and 1 atm the final density in $g/c{m}^3$ is

$\begin{array}{l}{T}_{initial}=380K\\ P_{initial}=722mmofHg\\ d_{initial}=1g/c{m}^3\end{array}$

$\begin{array}{l}{T}_{finial}=100K\\ P_{finial}=760mm\\ d_{finial}\end{array}$

For ideal gas  $\frac{d_1{T}_1}{P_1}=\frac{d_2{T}_2}{P_2}$
$\begin{array}{l}=\frac{1\times 380}{722}=\frac{d_2\times 100}{760}\\ =d_2=4\end{array}$