Answer
$i=-13cm$
Work Step by Step
Use the lens equation $$\frac{1}{f}=\frac{1}{i}+\frac{1}{p}$$ to solve for $i$. $$\frac{1}{i}=\frac{1}{f}-\frac{1}{p}$$ $$\frac{1}{i}=\frac{p}{pf}-\frac{f}{pf}$$ $$\frac{1}{i}=\frac{p-f}{pf}$$ $$i=\frac{pf}{p-f}$$ Substitute known values of $p=+8.0cm$ and $f=+20cm$ (see explanation in answer 79a) to get an image distance of $$i=\frac{(8.0cm)(20cm)}{8.0cm-20cm}=-13cm$$
