The hollow pipe and the solid shaft are coupled using a $6mm$ diameter shear pin $($cross$-$section shown below$).$

A$)$ If the shear strength of the pin is $400$MPa, determine the maximum allowable torque $T$ which can be applied assuming a safety factor against shear of $2\mathrm{.}5.$

B$)$ Determine the maximum shear stress in the solid shaft.

C$)$ Determine the shear stress on the inside surface of the hollow pipe.