4. A hollow steel shaft 3 m long must transmit a torque of 25 kN m. The total angle of twist in this length is not to exceed 2.5° and the allowable shearing stress is 90 MPa. If the modulus of rigidity of steel is 85 Gpa, determine a) The outside diameter, (Do) of the shaft and b) The inside diameter (Di) of the shaft. [10 marks]