If ${{\mathrm{E}}^0}_{{\mathrm{A}}^{+}/{\mathrm{A}}_{\left(\mathrm{s}\right)}}=1.69 \mathrm{V}$ and ${{\mathrm{E}}^0}_{{\mathrm{A}}^{+3}/{\mathrm{A}}_{\left(\mathrm{s}\right)}}=1.4 \mathrm{V}$ then ${{\mathrm{E}}^0}_{{\mathrm{A}}^{+3}/{\mathrm{A}}^{+}}$ will be :

$$\begin{gathered} {{\mathrm{E}}^0}_{{\mathrm{A}}^{+}/{\mathrm{A}}_{\left(\mathrm{s}\right)}}=1.69 \mathrm{V} \\ {{\mathrm{E}}^0}_{{\mathrm{A}}^{+3}/{\mathrm{A}}_{\left(\mathrm{s}\right)}}=1.4 \mathrm{V} \\ {\mathrm{A}}^{3+}\underset{(\mathrm{n}=2)}{\overset{(∆{\mathrm{G}}_1)}{\to }}{\mathrm{A}}^{+}\underset{(\mathrm{n}=1)}{\overset{(∆{\mathrm{G}}_2)}{\to }}\mathrm{A}\left(\mathrm{s}\right) \\ {\mathrm{A}}^{3+}\underset{ }{\overset{(∆{\mathrm{G}}_3=∆{\mathrm{G}}_1+∆{\mathrm{G}}_2)}{\to }}\mathrm{A}\left(\mathrm{s}\right) \end{gathered}$$

${\mathrm{A}}^{3+}+2{\mathrm{e}}^{-}\to {\mathrm{A}}^{+}; \mathrm{n} = 2$ (number of electrons)

${\mathrm{A}}^{+}+{\mathrm{e}}^{-}\to \mathrm{A}\left(\mathrm{s}\right); \mathrm{n} = 1$(number of electrons)

Here,

$$\begin{gathered} ∆{\mathrm{G}}_3=∆{\mathrm{G}}_1+∆{\mathrm{G}}_2 (\because ∆\mathrm{G}=-\mathrm{nFE}°) \\ – 3 \times \mathrm{F}\times 1.4 = –2 \times \mathrm{F}\times \mathrm{E}°+ \left(– 1 \times \mathrm{F}\mathbf{\times } 1.69\right) \\ – 4.2 = – (2\mathrm{E}° + 1.69) \\ 2 \mathrm{E}°= 4.2– 1.69 \\ \mathrm{E}°=\frac{2.51 \mathrm{V}}{2} \\ \mathrm{E}°=1.255 \mathrm{V} \end{gathered}$$