An electromagnet is made by inserting a soft iron core into a solenoid. The solenoid has 1800

Question:

An electromagnet is made by inserting a soft iron core into a solenoid. The solenoid has 1800 turns, radius 2.0 cm, and length 15 cm. When 2.0 A of current flows through the solenoid, the magnetic field inside the iron core has magnitude 0.42 T. What is the relative permeability kB of the iron core?