$V \propto P\,{r^4}$($\eta \;{\rm{and }}l$ are constants)
$\frac{{{V_2}}}{{{V_1}}} = \left( {\frac{{{P_2}}}{{{P_1}}}} \right)\;{\left( {\frac{{{r_2}}}{{{r_1}}}} \right)^4}$= $2 \times {\left( {\frac{1}{2}} \right)^4}$= $\frac{1}{8}$
${V_2} = \frac{Q}{8}$


