c
$u _{\max }=\frac{1}{2} m \omega^2 A ^2=25\,J$
$KE \text { at } \frac{ A }{2}=\frac{1}{2} mv _1^2=\frac{1}{2} m \omega^2\left( A ^2-\frac{ A ^2}{4}\right)$
$KE =\frac{1}{2} m \omega^2 \frac{3 A ^2}{4}=\frac{3}{4}\left(\frac{1}{2} m \omega^2 A ^2\right)$
$KE =\frac{3}{4} \times 25=18.75\,J$