Solid shaft $AC$ is fixed from left end $A.$ Two torques are applied at points $B$ and $C$ as shown.

a$)$ Determine maximum shear stress at segment $AB$ and $BC.$

b$)$ Determine angle of rotation at free end $C$ about the fixed end $A.$

$d_{AB}=40mm,d_{BC}=30mm,$ Shear Modulus $G=75$GPa.