yeaah i tried using magnification=image/object ratio thing. But idk why in the solution they have taken the length of image as 50. I agree with taking 20 as the length of object but i don't get the taking 50 as the length of image.
The location of the image is deterimined by drawing three special light rays from the object through the lens.
How should such rays be drawn?
Have you tried constructing such a drawing by say using a scale of 1 cm in the drawing is 2 cm in real life?