Modulus of Elasticity $=15{10}^6$ Modulus of Rigidity $=5\mathrm{.}5{10}^6$

a$.$ Calculate the shear stress at point $A($located on the inside surface of the hollow rod$)$ caused by the applied torques.

b$.$ Calculate the shear stress at point $B($located on the outside surface of the solid rod$)$ caused by the applied torques.

c$.$ Determine the magnitude and direction $($show with a curved arrow$)$ of the angle of twist at the free end of the hollow rod caused by applying the two torques.