L$4$ A solid bar ABC made of two concentric circular solid sections $AB$ and $BC$ is subjected to a torque of $1kNm$ at its free end $C$ and a torque of $3kNm$ at $B($both in the same direction$).$ Section $AB$ is $200mm$ in length and has diameter $100mm.$ Section BC is $400mm$ in length and has diameter $50mm.G=70k\frac{N}{m}{m}^2$ throughout. Calculate the maximum shear stress in the bar and the angle of twist at its free end.