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