The assembly is made of A-36 steel. Determine the angle of twist at C - 5-55

5-55. The assembly is made of A-36 steel and consists of a solid rod 20 mm in diameter fixed to the inside of a tube using a rigid disk at B. Determine the angle of twist at C. The tube has an outer diameter of 40 mm and wall thickness of 5 mm.
All problems from Mechanics of Materials Eleventh Edition by Russell C. Hibbeler.