How do you calculate relative permeability?

An iron ring of 420 mm mean diameter and 1.6 x 10^-3 m^2 cross-sectional area, has 1000 turns of wire wound uniformly around it. If a current of 1.4 A in the wire produces a magnetic flux of 4 x 10^-3 Wb in the iron core, estimate the relative permeability of the iron