Answer
The focal length objective is $~~2.0~mm$
Work Step by Step
We can find the focal length objective:
$M = -\frac{L}{f_{obj}}\cdot \frac{25~cm}{f_{eye}}$
$f_{obj} = -\frac{L}{M}\cdot \frac{25~cm}{f_{eye}}$
$f_{obj} = -\frac{20~cm}{-500}\cdot \frac{25~cm}{5.0~cm}$
$f_{obj} = 0.20~cm$
$f_{obj} = 2.0~mm$
The focal length objective is $~~2.0~mm$
