Two shafts are connected end to end by means of a flanged coupling in which there are 12 bolts, the pitch circle diameter being 30cm. The maximum shear stress is limited to 60N/mm2 in the shafts and 25N/mm2 in the bolts. If one shaft is solid of 6cm diameter and the other is hollow of 12cm external diameter, calculate the internal diameter of the hollow shaft and the bolt diameter so that both shafts and the coupling are all equally strong in torsion