Problem on Torsion/calculation of Diameter of shaft by strength and stiffness/Strength of Materials

Problem on Torsion/calculation of Diameter of shaft by strength and stiffness/Strength of Materials Problem: A solid shaft is subjected to a torque of 1.6 KN-m. Find the necessary diameter of the shaft, if the allowable shear stress is 60 MPa. The allowable twist is 1° for every 20 diameters length of the shaft. Take C = 80 GPa.