b
$\mathrm{E}=\frac{1}{2} \mathrm{m} \omega^{2} \mathrm{A}^{2} \Rightarrow \mathrm{E}=\frac{1}{2} \mathrm{m}(2 \pi \mathrm{f})^{2} \mathrm{A}^{2}$
$\Rightarrow \mathrm{A}=\frac{1}{2 \pi \mathrm{f}} \sqrt{\frac{2 \mathrm{E}}{\mathrm{m}}}$
Putting $\mathrm{E}=\mathrm{K}+\mathrm{U}$ we obtain,
$A=\frac{1}{2 \pi\left(\frac{25}{\pi}\right)} \sqrt{\frac{2 \times(0.5+0.4)}{0.2}} \Rightarrow A=0.06$