Q.
The magnetic field (B) inside a long solenoid having n, turns per unit length and carrying current I when iron core is kept in it is (μ0= permeability of vacuum, χ= magnetic susceptibility)
Magnetic field due to solenoid =μ×n×I
where μ= permeability n= turn density I= current
Also μ=μ0(1+χ)
where χ= magnetic susceptibility Bsolenoid =μ0nI(1+χ)