Determine the angle of twist of gear A relative to gear D if the shear modulus o

Determine the angle of twist of gear A relative to gear D if the shear modulus of elasticity G=75GPa.
diagram attached
The A-36 steel axel is made from tube AB and CD and a solid section BC. It is supported on smooth bearings that allow it rotate freely. The gear fixed to its ends are subjected to 90 Nm torques. The tubes AB and CD have an outer diameter of 30mm and inner diameter of 20mm. The solid section BC has a diameter of 40mm.
Determine the angle of twist of gear A relative to gear D if the shear modulus of elasticity G=75GPa.
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