\begin{aligned} & \mathrm{Cu}^{2+}+e^{-} \quad \rightarrow \mathrm{Cu}^{+} ; \\ & E_1^{\circ}=0.15 \mathrm{~V} ; \Delta G_1^{\circ}=-n_1 E_1^{\circ} F \\ & \mathrm{Cu}^{+}+e^{-} \rightarrow \mathrm{Cu} ; \\ & E_2^{\circ}=0.50 \mathrm{~V} ; \Delta G_2^{\circ}=-n_2 E_2^{\circ} F \\ & \hline \mathrm{Cu}^{2+}+2 e^{-} \quad \rightarrow \mathrm{Cu} ; E^{\circ}=? ; \Delta G^{\circ}=-n E^{\circ} F \\ & \Delta G^{\circ}=\Delta G_1^{\circ}+\Delta G_2^{\circ} \\ & \text { or }-2 E^{\circ} F=-1 F \times 0.15+(-1 F \times 0.50) \\ & \text { or }-2 E^{\circ} \mathrm{F}=-0.15 \mathrm{~F}-0.50 \mathrm{~F} \\ & \text { or }-2 F E^{\circ}=-F(0.15+0.50) \\ & \therefore \quad E^{\circ}= \frac{0.65}{2}=0.325 \mathrm{~V} \end{aligned}