An iron ring of mean length 50 cm and csa 0.8 cm2 is wound with a coil of 350 turns. A current of 0.5 A through the coil produces a flux density of 0.6 T in the ring. Calculate
(a) the relative permeability of the ring,
(b) the inductance of the coil, and
(c) the value of the induced emf if the current decays to 20% of its original value in 0.01 s, when the current is switched off .