## Fundamentals of Physics Extended (10th Edition)

$p=20cm$
To find the value of $p$, use the lens equation $$\frac{1}{f}=\frac{1}{p}+\frac{1}{i}$$ Solving for $p$ yields $$\frac{i}{if}-\frac{f}{if}=\frac{1}{p}$$ $$\frac{i-f}{if}=\frac{1}{p}$$ $$p=\frac{if}{i-f}$$ Substituting known values of $f=-20cm$ and $i=-10cm$ yields a p value of $$p=\frac{(-10cm)(-20cm)}{-10cm+20cm}=20cm$$