A hollow steel shaft is to transmit 5400 N .m of torque and is to be sized

Question:

A hollow steel shaft is to transmit 5400 N .m of torque and is to be sized so that the torsional stress does not exceed 150 MPa.
(a) If the inside diameter is three-fourths of the outside diameter, what size shaft should be used? Use preferred sizes.
(b) What is the stress on the inside of the shaft when full torque is applied?