Finding magnetic field inside closed iron bar
Current is a peak of 25 amps and, you have the turns (72) so, you have the MMF (magneto motive foce) because that equals amps x turns. So MMF = 1800 (peak).

You also have the length of the core and you can reasonably assume that 95%+ of the magnetic field flows through the core and gap. 

Magnetic field strength (H) = MMF/length of core. Hence, H = 1000 ampere turns per metre. Looks like the numbers are working out nicely.

Next you have to work out effective permeability i.e. the permeability with the gap and this formula is as shown on this calculator: -



You need to multiply this by the permeability of a vacuum (air) to get a real number so, a relative permeability of 300 becomes an absolute permeability of 0.000377.

B = H x permeability hence B (peak) = 0.377 teslas.

Area and resistance (although quoted in the question), are not used in this calculation.