$4\mathrm{.}7$ A $2kN.m$ torque is applied to a $3m$ long steel angle with a $20020013$ cross section. From tabulated properties for 'equal angles', the gross cross sectional area of the section is $5090m{m}^2.$ Knowing that $G=77$GPa, determine $($a$)$ the maximum shearing stress along line $m-m,($b$)$ the angle of twist. $[90\mathrm{.}8$ MPa, $[$:$15\mathrm{.}6\}$