This one is giving me a headache. I thought I had the right solution after a long time of scrolling back and forth in the physics book... I think the biggest problem for me is how do I use that 1.26x10-6 N/A^2 for the permeability of free space. If you can answer this you might wanna post the whole procedure of solving this problem, cause my problem might be that everything I'm trying is wrong! A toroidal solenoid has a cross-sectional area of 0.420 cm^2 , a mean radius of 12.6 cm, and 1850 turns. The space inside the windings is filled with a ferromagnetic material that has a relative permeability of 500. Calculate the inductance of the solenoid. (You can ignore the variation of the magnetic field across the cross section of the toroid.) Use 1.26×10-6 N/A^2 for the permeability of free space. Please help me. I'm not trying to get someone to do my homework for me, I just didn't see any other alternatives after all this time of trying! Thank you.