Q$2-$ Statically indeterminate torsion $(25$ points$)$

A statically indeterminate stepped compound solid shaft ACB is fixed at ends A and $B,$ and loaded by a torque $T_C=5kNm$ at point $C.$ Segment $AC$ is made of copper with $G_{\text{c}\text{e}\text{p}\text{p}\text{e}\text{r}}=50$GPa, having a length of $300$ mm and a diameter of $100$ mm $,$ while segment CB is made of steel with $G_{\text{t}\text{h}\text{e}\text{l}}=80$GPa, having a length of $350$ mm and a diameter $d.$ Determine diameter $d$ of segment $CB$ such that both segments have the same shear stress.