An insulating ball of radius $R($with negligible dielectric properties$)$ has a spherical cavity of radius $\frac{R}{2}$ inside with its center is of distance $\frac{R}{2}$ from the original center of the ball. The remaining part of the ball is charged with uniform charge distribution ${}_0.$ What is the potential at the center of the cavity? $($Hint: First calculate the potential inside a uniformly charged ball by integrating vec$(E)$ directly, $($:$V=-{\displaystyle \underset{}{\overset{r}{}}}(vec(E))*vec(dl)\}.$