\(T = 2\pi \sqrt {\frac{1}{{MB}}} \)
where,
\(\mathrm{I}=\) Moment of inertia of the magnet about the axis of rotation
\(\mathrm{M}=\) Magnetic moment of the magnet
\(\mathrm{B}=\) Uniform magnetic field
As the \(I\), \(B\) remains the same
\(\therefore \) \(\mathrm{T} \propto \frac{1}{\sqrt{\mathrm{B}}} \text { or } \frac{\mathrm{T}_{2}}{\mathrm{T}_{1}}=\sqrt{\frac{\mathrm{B}_{1}}{\mathrm{B}_{2}}}\)
According to given problem.
\({\mathrm{B}_{1}=24 \,\mu \mathrm{T}}\)
\({\mathrm{B}_{2}=24\, \mu \mathrm{T}-18\, \mu \mathrm{T}=6\, \mu \mathrm{T}}\)
\({\mathrm{T}_{1}=2\, \mathrm{s}}\)
\(\therefore \) \({\mathrm{T}_{2}=(2\, \mathrm{s}) \sqrt{\frac{(24\, \mu \mathrm{T})}{(6\, \mu \mathrm{T})}}=4 \,\mathrm{s}}\)