Answer
$f=+80.0cm$
Work Step by Step
Use the magnification equation $$m=-\frac{i}{p}$$ to solve for $i$. $$i=-mp$$ Substitute this value into the lens equation. $$\frac{1}{f}=\frac{1}{p}+\frac{1}{i}$$ $$\frac{1}{f}=\frac{1}{p}-\frac{1}{mp}$$ Solve for $f$: $$\frac{1}{f}=\frac{m}{mp}-\frac{1}{mp}$$ $$\frac{1}{f}=\frac{m-1}{mp}$$ $$f=\frac{mp}{m-1}$$ Substitute known values of $m=1.25$ and $p=16cm$ to get a focal length of $$f=\frac{(1.25)(16cm)}{1.25-1.00}=80.0 cm$$
