A mild steel shaft of 15 cm diameter is subjected to a bending moment of 20000 kgf/cm and a torque T. If the yield point of the steel in tension is 2000 kgf / cm2, find maximum value of this torque without causing yielding of the shaft according to
(i) the maximum principal stress,
(ii) the maximum shear stress and
(iii) the maximum distortion strain energy theories of yielding.