A parallel beam of rays is incident on a convergent lens with a focal length of $40cm$. Where should a divergent lens with a focal length of $15cm$ be placed for the beam of rays to remain parallel after passing through the two lenses? Draw a ray diagram

If an incident ray is approaching focus of a divergent lens then they become parallel to principal axis.

So if convergent and divergent lens focus coincide then the ray will become parallel to principal axis.

Hence $x+15=40$

$x=25cm$

Hence divergent lens should be 25 cm behind convergent lens.