The shear modulus (G) of steel and a quasi-isotropic carbon fiber- epoxy is 78 and 17 GPa, respectively. The mean diameter (D) of a thin-walled steel torque tube is 25 mm and its wall thickness (1) is 3 mm. Knowing that the torsional stiffness of a thin-walled tube is propor- tional to D'IG, calculate: 1. Mean diameter of a composite tube that has the same torsional stiffness and wall thickness as the steel tube 2. Wall thickness of a composite tube that has the same torsional stiffness and mean diameter as the steel tube 3. Difference in weight (in percentage) between the steel tube and the composite tube in each of the previous cases, assuming equal length for both tubes