A long, hollow, cylindrical conductor (having inner radius 6.0 mm and outer radius 12.0 mm) carries a current of 18 A distributed equally across its cross section. A long thin wire which is coaxial with the cylinder carries a current of 18 A in the opposite direction. Determine the magnitude of the magnetic field:
(i) 3.0 mm from the central axis
(ii) 9.0 mm from the central axis
(iii) 15.0 mm from the central axis