Answer
$r=4.5 m$
Work Step by Step
We know that
$V=K\frac{q}{r}$
Thus, it follows that:
$r=K\frac{q}{V}$
We plug in the known values to obtain:
$r=9\times10^9\times\frac{1.5\times10^{-8}}{30}$
$r=4.5 m$
Thus the equipotential surface is a sphere with a radius of $r=4.5 m$
