$K . E .=\frac{1}{2} m a^{2} \omega^{2} \cos ^{2} \omega t, \quad T . E .=\frac{1}{2} m a^{2} \omega^{2}$

Given $\mathrm{K} . \mathrm{E}=0.75 \mathrm{T.E.}$

$\Rightarrow 0.75=\cos ^{2} \omega t \Rightarrow \omega t=\frac{\pi}{6}$

$\Rightarrow t=\frac{\pi}{6 \times \omega} \Rightarrow t=\frac{\pi \times 2}{6 \times 2 \pi} \Rightarrow t=\frac{1}{6} s$