Answer
$M = -22$
Work Step by Step
See Figure 25-20. When the object and the image are both far away, the separation of the 2 lenses is the sum of their focal lengths.
$$L=f_o+f_e$$
$$ f_e =L-f_o =82.0-78.5cm=3.5cm$$
Use equation 25-3 to find the magnification of the telescope.
$$M=-\frac{f_o}{f_e}=-\frac{78.5cm}{3.5cm}\approx-22$$
