(b) Here, $n_{1} = 10\ $oscillation per min
$\delta_{1} = 45{^\circ},\ B_{1} = 0.707\ $CGS units
$n_{2} = ?,\ \delta_{2}60{^\circ},\ B_{2} = 0.5$ CGS units
$\frac{n_{2}}{n_{1}} = \sqrt{\frac{H_{2}}{H_{1}}} = \sqrt{\frac{B_{2}\cos\delta_{2}}{B_{1}\cos\delta_{1}}}$
$\frac{n_{2}}{n_{1}} = \sqrt{\frac{0.5\cos{60{^\circ}}}{0.707\ \cos{45{^\circ}}}}\ = \sqrt{\frac{0.5 \times 1/2}{0.5 \times \sqrt{2} \times \frac{1}{\sqrt{2}}}}$
$\frac{n_{2}}{10} = \frac{1}{\sqrt{2}} \Longrightarrow n_{2} = \frac{10}{\sqrt{2}} = 7.07 \approx 7$