Answer
See answers.
Work Step by Step
Equation 25–3 links the magnification to the 2 focal lengths. Solve for the focal length of the objective lens.
$$M=-\frac{f_o}{f_e}$$
$$f_o=-Mf_e$$
The distance between the two lenses is the sum of the 2 focal lengths. Combine this with equation 25–3 and solve for the focal lengths.
$$\ell=f_o+f_e=(1-M)f_e$$
The magnification for an astronomical telescope is negative.
$$f_e=\frac{\ell}{1-M}=\frac{28cm}{1-(-7.5)}=3.3cm$$
$$f_o=-Mf_e=-(-7.5)( 3.3cm)=25 cm$$