A magnetic circuit comprises three parts in series, each of uniform cross-sectional area (c.s.a.). They  are: (a) a length of 80 mm and c.s.a. 50 mm2, (b) a length of 60 mm and c.s.a. 90 mm2, (c) an airgap of length 0.5 mm and c.s.a. 150 mm2. A coil of 4000 turns is wound on part (b), and the flux density in the airgap is 0.30 T. Assuming that all the flux passes through the given circuit and that the relative permeability μr is 1300, estimate the coil current to produce such a flux density.