The best vaccum so far attained in laboratory is $10^{-10}mm$ of Hg. How many molecules of gas remain per $cm^3$ at $20^{\large\circ}C$ in this vaccum?

Question

$\begin{array}{1,1}(a)\;3.29\times10^4 \;molecules\\(b)\;3.29\times10^6 \;molecules\\(c)\;3.29\times10^{10} \;molecules\\(d)\;3.29\times10^5 \;molecules\end{array}$