$20 dm^3$ of $SO_2$ diffuses through a porous partition in 60 Seconds. What Volume be $O_2$ will diffuse under similiar conditions in 30 second?

Question

$\begin{array}{1 1} 12.3\;dm^3 \\ 14.4\;dm^3 \\ 10.13\;dm^3 \\ 11.13\;dm^3 \end{array} $