What formula did you use, and the answer?

Question

The intensity at a distance of 4 ft from an ordinary light bulb is ______ as great as that at a distance of 12 ft from the same bulb.
A. 43
B. 15
C. -18
D. 1/12
E. 5
F. 9
G. 18

Answer 1

The intensity drops off with the square of the increasing distance.
1 ft >> I
2 ft >>I/4
3 ft >>I/9 etc