$5\mathrm{.}4$ The cross section of a closed, thin$-$walled, semicircular steel tube is transmitting a torsional moment of $500$N$*$m$.$ The wall thickness of the tube is $3$ mm$,$ and the radius between wall centers is $40$ mm $.$ Determine the average shear stress in the tube and the angle of twist over a length of $300$ mm $.$ For steel let E$=210$GPa and v$=0\mathrm{.}29($Poission Ratio$).$