#### Answer

r $\approx$ 6.7cm

#### Work Step by Step

Given A metal circular disk whose area is 143 $cm^{2}$ is used as a knockout on an electrical service in a factory
The area A of a circle whose radius has length r is given by
A = $\pi r^{2}$
143 = $\pi r^{2}$
$r^{2}$ = $\frac{143 * 7}{22}$ = 45.5
r = $\sqrt 45.5$
r = 6.746 cm
r $\approx$ 6.7cm