Shaft angle of twist limit and implied shear stress

[loststudent123]

A rule often used in shaft design states that the angle of twist shall not exceed 1 on a length equal to 20 diameters. What stress in the material does this imply if the modulus of rigidity is equal to 80 × 103 N/mm2 ?
was wondering if anybody had an insight on how to do this ?

have tried a few ways but keep being left with 2 unknowns

 

[SteamKing]

Please use the HW Template and fill out all three sections, including the one which includes your attempts at solution.