# Chapter 9 - Section 9.4 - Variation and Problem Solving - Exercise Set - Page 665: 43

$8$ feet

#### Work Step by Step

Let $I$ be the intensity of light. $I=\frac{k}{x^2}$ $I=80; x=2$ $80=\frac{k}{(2)^2}$ $k=320$ $I=\frac{320}{x^2}$ $I=5$ $5=\frac{320}{x^2}$ $x^2=\frac{320}{5}$ $x^2=64$ $x=8$ feet

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