## Precalculus (6th Edition) Blitzer

Consider $I$ to be the intensity and $d$ to be the distance of the source. We have $I=\dfrac{k}{d^2}$ ....(1) so, $20=\dfrac{k}{225}$ This implies that $k=4500$ Equation (1) becomes: $I=\dfrac{4500}{(10)^2}=45$ Therefore, the intensity of a light at a distance 10 feet from the source is 45 foot-candles.