The time period of oscillations of magnet

$T=2\pi \sqrt{\left(\frac{I}{M{B}_H}\right)} \dots \left(\mathrm{i}\right)$

Where $I=$ moment of inertia of magnet

$=\frac{m{L}^2}{12}$ (m, being the mass of magnet)

When the three equal parts of magnet are placed on one another with their like poles together, then

$I^{\prime }=\frac{1}{12}\left(\frac{m}{3}\right)\times {\left(\frac{L}{3}\right)}^2\times 3$

$=\frac{1}{12}\frac{m{L}^2}{9}=\frac{I}{9}$

and $M^{\prime }=$ pole strength $\times \frac{L}{3}\times 3=M$

Hence,

$\Rightarrow T^{\prime }=\frac{1}{3}\times T$

$\Rightarrow T^{\prime }=\frac{150}{3}=50 \mathrm{ms}$