$4 \frac{d^2 x}{d t^2}+320 x=0$
$4 a=-320 x$
$a=-80 x$
Since $a=-\omega^2 x$ in $S.H.M.$
$80=\omega^2$
$\sqrt{16 \times 5}=\omega$
or $\omega=4 \sqrt{5}$
$T=\frac{2 \pi}{\omega}=\frac{2 \pi}{4 \sqrt{3}}=\frac{\pi}{2 \sqrt{5}} s$
$x = 0.01\cos \left( {\pi \,t + \frac{\pi }{4}} \right)$
The frequency of the motion will be

