Answer
$\overline\omega=1.2\times10^4rad/s$
Work Step by Step
The mandible's angular displacement is $$\theta=\Big(\frac{90^o}{360^o}\Big)\times(2\pi rad)=\frac{\pi}{2}rad$$
The time it takes is $t=1.3\times10^{-4}s$. The average angular velocity is $$\overline\omega=\frac{\theta}{t}=1.2\times10^4rad/s$$
