Structural aluminum tubing of 2.5×4-in. rectangular cross section was fabricated by extrusion. Determine the shearing stress in each of the four walls of a portion of such tubing when it is subjected to a torque of 24 kip.in., assuming (a) a uniform 0.160-in. wall thickness (Fig. a), (b) that, as a result of defective fabrication, walls AB and AC are 0.120-in. thick, and walls BD and CD are 0.200-in. thick (Fig. b).