Four identical lenses are kept one beside the other on the same optical axis as shown in the figure. The right surface of rightmost lens is silvered. Focal length of each lens is 20 cm and radius of silvered surface is 20 cm. The focal length of the combined system is

-2 cm

$\frac{1}{F_e}=\frac{1}{f_m}-\frac{2}{f_{\ell}} \times 4=\frac{1}{-10}-\frac{2}{5}=\frac{-1}{2}$

$F_e$ = -2 cm