Answer
$i=20cm$
Work Step by Step
Use the lens equation $$\frac{1}{f}=\frac{1}{p}+\frac{1}{i}$$ to solve for $i$. $$\frac{1}{i}=\frac{1}{f}-\frac{1}{p}$$ $$\frac{1}{i}=\frac{p}{pf}-\frac{f}{pf}$$ $$\frac{1}{i}=\frac{p-f}{pf}$$ $$i=\frac{pf}{p-f}$$ Since the lens is converging, the focal length is positive (f=+10cm) Substituting known values of $f=+10cm$ and $p=+20cm$ yields an image distance of $$i=\frac{(20cm)(10cm)}{20cm-10cm}=20cm$$
