${{U}_{2}}=\frac{1}{2}CV_{2}^{2}\Rightarrow 40=\frac{1}{2}\times 20\times V_{2}^{2}\Rightarrow V_{2}^{2}=4\Rightarrow {{V}_{2}}=2V$ ${{V}_{2}}=2{{V}_{1}}\Rightarrow {{V}_{1}}=1V$