What is the magnification of an astronomical telescope whose objective lens has a focal length of 82 cm, and whose eyepiece has a focal length of 2.8 cm?

The negative ratio of focal length of the objective to the focal length of the eyepiece give the focal length of the telescope

$M=-\frac{f_0}{f_e}$

Here, $f_0$ is focal length, M is magnification and $f_e$ is eyepiece of focal length

Substitute $2.8$ cm for $f_e$, and $82cm$ for $f_0$

$M=-\frac{82}{2.8}=-29X$