$\because \,\,C{u^{ + 2}}\, + \,\,{e^ - }\, \to \,\,C{u^ + }\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,{E^ o }\, = \,\,  \,\,0.153\,\,volt$

$\therefore \,\,C{u^ + }\, \to \,\,C{u^{ + 2}}\, + \,\,{e^ - }\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,E_2^o \,= \,- \,0.153\,volt,\,\,{n_2}\, = \,1$

$C{u^ + }\, + \,\,{e^ - }\, \to \,\,Cu\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,E_3^o \,\, = \,\,??,\,\,{n_3}\, = \,\,1$

$\therefore \,{\mkern 1mu} {\mkern 1mu} E_3^o{\mkern 1mu} {\mkern 1mu}  = \frac{{{\mkern 1mu} {n_1}E_1^o{\mkern 1mu}  + {\mkern 1mu} {\mkern 1mu} {n_2}E_2^o}}{{{n_3}}}$

$ = \,\,\frac{{2\,\, \times \,\,0.337\,\, + \,\,1\,\, \times \,\,( - 0.153)}}{1}$

$ = \,\,\,0.521\,\,volt$