Answer
$f=+16cm$
Work Step by Step
Solve the lens equation $$\frac{1}{f}=\frac{1}{p}+\frac{1}{i}$$ for focal length $f$: $$\frac{1}{f}=\frac{i}{ip}+\frac{p}{ip}$$ $$\frac{1}{f}=\frac{i+p}{ip}$$ $$f=\frac{ip}{i+p}$$ Substitute known values of $i=+28cm$ and $p=+40cm$ yields a focal length of $$f=\frac{(+28cm)(+40cm)}{28cm+40cm}=+16cm$$
