The coercivity of a small magnet where the ferromagnet gets demagnetized is $$3 \times {10^3}A{m^{ - 1}}.$$   The current required to be passed in a solenoid of length $$10\,cm$$  and number of turns 100, so that the magnet gets demagnetized when inside the solenoid, is:

Solution :
Magnetic field in solenoid $$B = {\mu _0}ni$$
$$\eqalign{ & \Rightarrow \frac{B}{{{\mu _0}}} = ni\,\,\,\left( {{\text{where}}\,n = {\text{number of turns per unit length}}} \right) \cr & \Rightarrow \frac{B}{{{\mu _0}}} = \frac{{Ni}}{L} \cr & \Rightarrow 3 \times {10^3} = \frac{{100i}}{{10 \times {{10}^{ - 2}}}} \cr & \Rightarrow i = 3A \cr} $$