inductance calculation

the question is:
The steel winding with the number of coils w= 200 is put on thr ring-like closed
core in regular intervals.The section is rectangular. The external diameter of a ring
D=16 sm, the interval diameter d=10 sm, thickness b=4 sm.Magnetic induction B in
the core have to be equal 1 T1. How will current and magnetic resistance changes,
if the nonmagnetic (air) 'interval b=0.5 mm would be made. Magnetic flux has to be
constant.Define the relative magnetic permeability and magnetic resistance of the
ferromagnetic steel and sheet steel 1512 cores, if the magnetic flux in the interval
=1.2x1/1000 Wb. Dependence B(H) for steel is approximate to a such data:
0,2(80)0,4(185)0,6(300)0,8(460)1,0(750)1,2(1500)

my question is how to solve this problem, which formulas can calculate this...