Answer
The Illumination decreases four times
Work Step by Step
The Illumination from a light source varies inversely as the square of the distance from a light source.
$I_{1}=\frac{k}{r^2}$. whereas $I$ is the Illumination, $r^2$ is the square of the distance from a light source and $k$ a constant.
$I_{1}=\frac{k}{15^2}$,
$I_{1}=\frac{k}{225}$,
$I_{2}=\frac{k}{30^2}$,
$I_{1}=\frac{k}{900}$,
$\frac{I_{2}}{I_{1}}=\frac{\frac{k}{900}}{\frac{k}{225}}=\frac{225}{900}=\frac{1}{4}$,
$4I_{2}=I_{1}$,
$I_{2}=\frac{I_{1}}{4}$.
We can see that when the distance from the light source doubles, the Illumination decreases four times.
