Answer
$i=60cm$
Work Step by Step
Use the lens equation $$\frac{1}{f}=\frac{1}{p}+\frac{1}{i}$$ to solve for $i$. $$\frac{1}{i}=\frac{1}{f}-\frac{1}{p}$$ $$\frac{1}{i}=\frac{p}{fp}-\frac{f}{fp}$$ $$\frac{1}{i}=\frac{p-f}{pf}$$ $$i=\frac{pf}{p-f}$$ Substituting known values of $f=20cm$ and $p=30cm$ yields an image distance of $$i=\frac{(30cm)(20cm)}{30cm-20cm}=60cm$$
