$1)$ The solid circular shaft is fixed at one end and subjected to a torque of $4000$m ft$.$ Ib at the free end. Determine the $($a$)$ max. shear stress in the steel, $($b$)$ max. shear stress in the aluminum, and $($c$)$ angle of twist at the free end. Use Gstee $12$x$10$ psi and Ga $-4$x$106$ psi. $(20$ pts$)$