a
$y=r \sin \omega t$
$12.5=25 \sin \frac{2 \pi}{3} \times t \quad\left(\because \omega=\frac{2 \pi}{\mathrm{T}}\right)$
$\frac{\pi}{6}=\frac{2 \pi}{3} t$
$t=\frac{1}{4} \sec =0.25 \mathrm{sec}$
$t^{\prime}=2 t$
$=2 \times 0.25=0.5 \mathrm{sec}$
for either side