For hollow shaft $\left(1\right)$:

$$\begin{gathered} {\mathrm{J}}_1=\frac{\mathrm{\pi }}{32}\left({{\mathrm{D}}_1}^4-{{\mathrm{d}}_1}^4\right) \\ =\frac{\mathrm{\pi }}{32}\left[{(80 \mathrm{mm})}^4-{(65 \mathrm{mm})}^4\right] \\ =2,268,758 {\mathrm{mm}}^4 \end{gathered}$$

and

${\mathrm{c}}_1=\frac{80 \mathrm{mm}}{2}=40 \mathrm{mm}$

For solid shaft $\left(2\right)$:

$$\begin{gathered} {\mathrm{J}}_2=\frac{\mathrm{\pi }}{32}{{\mathrm{d}}_2}^4 \\ =\frac{\mathrm{\pi }}{32}{(80 \mathrm{mm})}^4 \\ =4,021,239 {\mathrm{mm}}^4 \end{gathered}$$

and

${\mathrm{c}}_2=\frac{80 \mathrm{mm}}{2}=40 \mathrm{mm}$