If the tube length of astronomical telescope is 105 cm and magnifying power is 20 for normal setting, calculate the focal length of the objective  

100 cm                                                                     

$\text{Length of an astronomical telescope is }L = f_0 + f_ e = 105cm$

$ M = \frac{f_0}{f_e} = 20 \Rightarrow f_0 = 20f_e $

$ 21f_e = 105cm  \Rightarrow f_e = 5cm , f_0 = 100cm.$