UPSEE 2017 Solved Paper

The light rays are coming from infinity and are incident on lens A parallel to the axis. It will form point image at F i.e. on focal point.
This F will act as source of light for lens B after refraction B, the emergent ray will parallel to axis of lens. Similarly, after refraction from C, they will converge at F. This F will act as a source. After refraction from D, the light rays will emerged parallel to the axes of lens.
Now, from figure
AD=30+10+20+30+10
=100cm