Magnetism And Matter

A rod with a circular cross-section area $2$ cm$^2$ and length $40$ cm is wound uniformly with $400$ turns of an insulated wire. If a current $0.4$ A flows in the wire winding, the total magnetic flux produced inside the winding is $4\pi \times 10^{-6}$ Wb. The relative permeability of the rod is
(Given permeability of vacuum $\mu_0 = 4\pi \times 10^{-7}$ N m A$^{-2}$)