Question 1093805
The intensity of light (in foot-candle) varies inversely as the square of the distance (in feet) from the light source.
 The intensity of light 3 feet from the source is 72 foot-candles.
 How far away is the source if the intensity of light is 2 foot-candles. 
:
Intensity = k over distance
I = k/d^2
find k when I=72 and d=3
72 = k/3^2
72 = k/9
k = 9 * 72
k = 648
"How far away is the source if the intensity of light is 2 foot-candles?"
2 = 648/d^2
2d^2 = 648
divid2 both sides by 2
d^2 = 324
d = {{{sqrt(324)}}}
d = 18 ft