Answer
The focal length of the eyepiece should be $~~5.0~cm$
Work Step by Step
We can find the required focal length of the eyepiece:
$M = -\frac{L}{f_o}\cdot \frac{25~cm}{f_e}$
$f_e = -\frac{L}{f_o}\cdot \frac{25~cm}{M}$
$f_e = \frac{160~mm}{8.0~mm}\cdot \frac{25~cm}{100}$
$f_e = 5.0~cm$
The focal length of the eyepiece should be $~~5.0~cm$
