Cu²⁺ + 2e^– $\to$ Cu; E°₁ = 0.34 V .....(1)

Cu²⁺ + e^– $\to$ Cu⁺ ; E°₂ = 0.15 V.....(2)

Cu⁺ + e^– $\to$ Cu; E°₃ = ? .....(3)

$\therefore$ $\Delta$G^o₁ = -2$\times$0.34$\times$F

and $\Delta$G^o₂ = -1$\times$0.15$\times$F

and $\Delta$G^o₃ = -1$\times$E°₃$\times$F

Also, $\Delta$G^o₁ = $\Delta$G^o₂ + $\Delta$G^o₃

$\Rightarrow$ -0.68F = -0.15F - E°₃ $\times$ F

$\Rightarrow$ E°₃ = 0 68 - 0 15 = 0 53 V

$\therefore$ E°_cell = 0.53 - 0.15 = 0.38 V