a
$\ell > \ell_0 \rightarrow k=k_1$
$\ell < \ell_0 \rightarrow k=k_2$
Time period of oscillation,
$T=\pi \sqrt{\frac{ m }{ k _1}}+\pi \sqrt{\frac{ m }{ k _2}}$
$T=\pi \sqrt{\frac{0.1}{0.009}}+\pi \sqrt{\frac{0.1}{0.016}}$
$T =\frac{\pi}{0.3}+\frac{\pi}{0.4} \Rightarrow T=\frac{0.7}{0.12} \pi \Rightarrow T=5.83 \pi$
$T\approx 6 \pi$
So, $n=6$
