\[\begin{align}
& \frac{{{U}_{2}}}{{{U}_{1}}}={{(\frac{{{Q}_{2}}}{{{Q}_{1}}})}^{2}}\to \frac{{{U}_{1}}+90}{{{U}_{1}}}={{(\frac{125}{100})}^{2}}=\frac{25}{16}\to 16{{U}_{1}}+1440=25{{U}_{1}}\to {{U}_{1}}=\frac{1440}{9}=160\mu J \\
& {{U}_{1}}=\frac{1}{2}c{{V}^{2}}\to 160=\frac{1}{2}\times 5\times {{V}^{2}}\to V=8v \\
\end{align}\]