Answer
$9.2\,mm$
Work Step by Step
Lateral magnification $m=-\frac{s'}{s}$ where $s'$ is the image distance and $s$ is the object distance.
Using the formula $\frac{1}{f}=\frac{1}{s}+\frac{1}{s'}$, we get
$\frac{1}{9.0\,mm}=\frac{1}{s}+\frac{1}{40s}=\frac{41}{40s}$
$\implies s=\frac{41}{40}(9.0\,mm)=9.2\,mm$
