$v(t)=5-10 \cos (\omega t+ 60^{\circ}) \: V$
$i(t)=5 + X \cos (\omega t) \: A$
where $\omega =100 \: \pi \text{ radian}/s$. If the average power delivered to the circuit is zero, then the value of $X$ (in Ampere) is _______ (up to $2$ decimal places)