Question 666455
Use
(1) I = c/(d^2), where I is the intensity in foot-candles, c a constant of propotionality and d is the distance in feet.
First we find c using the known intensity at a known distance or
(2) I = c/30^2 = 79, giving
(3) c = 79*900
To find the intensity at 10 feet use (1) with c of (3) and get
(4) I at 10' = 79*900/10^2 or
(5) I at 10' = 79*9 = 711
Answer: The light intensity at 10' is 711 foot-candles.