$4.$ A thin$-$walled cylindrical tank with end caps, seen below, is acted on by an internal pressure $\backslash ($ p$=5\backslash )$ MPa, an axial tensile load $\backslash (\backslash$mathrm$\{$P$\}=75\backslash$mathrm$\{$kN$\}\backslash )$ and an unknown torque, T $.$ The outer radius is $65$ mm and the wall thickness is $5$ mm $.$ Calculate the allowable value of the torque T $,$ in kN $-$ m $,$ if the allowable tensile stress in the tank wall is $85$ MPa $.$