A hollow shaft with diameter ratio 0.7 is required to transmit 500kW at 300rpm with a uniform twisting moment. Allowable shear stress in the material is 65N/mm2 and the twist in a length of 2.4m is not to exceed one degree. Calculate the minimum external diameter of the shaft satisfying these conditions. Modulus of rigidity = 8.2´104 N/mm2.