Q$3(5$ Points$)$: A hollow circular shaft with an outer diameter of $50$ mm and inner diameter of $40$ mm $.$ The length is $1\mathrm{.}2$ m $($shown in the figure$).$ The shaft is subjected to a torque of $4\mathrm{.}0kN.m.$

Assuming that the shaft is made of an elastoplastic material with allowable shear stress of $300$ MPa and modulus of rigidity $G=77$GPa.

Determine the following:

$($a$)$ The maximum shear stress and shear strain.

$($b$)$ The angle of twist of the shaft.

I need some one to solve this problem