Answer
$r=+16cm$
Work Step by Step
Use the equation relating focal length and radius of curvature $$f=\frac{r}{2}$$ to solve for $r$; $$r=2f$$ We then substitute the known value of $f=+8.0cm$ (see answer 18b for explanation) to get a radius of curvature of $$r=2(+8.0cm)=+16cm$$
