A single-core concentric cable is to be manufactured for a 60kV, 50 Hz transmission system. The dielectric used is paper, which has a maximum permissible safe dielectric stress of 10MV/m rms. and a relative permittivity of 3.5. Calculate (a) the core and inner sheath radii for the most economical cable, (b) the capacitance per metre length and (c) the charging current per kilometre run.