Determined the effective focal length of the combination of two lenses of focal length 30 cm and 20 cm if they are placed 8.0 cm apart with their principal axes coincident. Does the answer depend on which side a beam a parallel light is incident ? Is the motion of effective focal length of system useful at all ?

Question

$\begin{array}{1 1} 416\;cm \\ 41.6\;cm \\ 420 \;cm \\ 22\;cm \end{array} $