On $13^{\text {th }}$ collision,
$\mathrm{m} \rightarrow {\mathrm{M}+12} ; \quad \mathrm{M}+13 \mathrm{m} \rightarrow \mathrm{V}$
$\mathrm{mu}=(\mathrm{M}+13 \mathrm{m}) \mathrm{v} \Rightarrow \mathrm{v}=\frac{\mathrm{mu}}{\mathrm{M}+13 \mathrm{m}}=\frac{\mathrm{u}}{15}$
$v=\omega A \Rightarrow \frac{u}{15}=\sqrt{\frac{K}{M-13 m}} \times A$
Putting value of $M, m, u$ and $K$ we get amplitude
$A=\frac{1}{15} \sqrt{\frac{75}{1}}=\frac{1}{\sqrt{3}}$
