Let me backup.
I have 4 questions about magnetic.
The first question is to find the field strength distance of a a straight conductor , 0,48 cm from the conductor. Current is 3 Ampere.
Question number two is to find how many turns in the drawing for the giving information. only given are Current, relative permeability , and length of the coil, the cross section is square. Outer and inner length are given.
I have a similar example in the book where they applied the formula A=B.D .
A is the cross section in m^2
B outer length
D inner length
then they applied HOPKINSON'S law, Ampere winding number Fm=I.N and the formula Reluctance in a circuit Rm= Iota( greek letter) / u.A