The parallel
incident beam appears to diverge from a point 416 cm on the
left of the centre of the two-lens system Clearly, the answer depends on which side of the lens system
the parallel beam is incident Further we do not have a simple lens
equation true for all u (and v) in terms of a definite constant of the
system  (the constant being determined by f1 and f2, and the separation
between the lenses) The notion of effective focal length, therefore,
does not seem to be meaningful for this system