A Galilean telescope is 27 cm long when focused to form an image at infinity. If the objective has a focal length of 30 cm, what is the focal length of the eye piece?

For the given Galilean telescope, (when the image is formed at infinity)
${\mathrm{f}}_0=30\mathrm{cm},\mathrm{L}=27\mathrm{cm}$
Since, $\mathrm{L}={\mathrm{f}}_0-\left|{\mathrm{f}}_{\mathrm{e}}\right|$
[Since, concave eyepiece lens is used in Galilean Telescope]
$\Rightarrow {\mathrm{f}}_{\mathrm{c}}={\mathrm{f}}_0-\mathrm{L}=30-27=3\mathrm{cm}$