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 10 cm 20 cm 25 cm

100 cm

$L=f_0+f_e=105cm$ and $M=\frac{f_0}{f_e}=20$ $⇒f_0=100cm$ and $f_e=5cm$