SSC JE Electrical Engineering 2015 Paper

The reluctance of a uniform magnetic circuit
$\mathit{R}=\frac{1}{{\mathit{\mu }}_0{\mathit{\mu }}_{\mathrm{r}}\mathrm{}\mathit{A}}$
L = length
${\mathit{\mu }}_0$ = permeability of vacuum
${\mathit{\mu }}_{\mathit{r}}$ = Relative permeability of material
A = Cross section area of circuit
Reluctance of air > Reluctance of iron because relative permeability of iron or > Relative permeability of air