it's unclear to me whether to interpret the point on the sample where the light rays converge as the focal length "f" or the image distance "i". The question interprets this as the image distance.
I think the sample light ray would converge at the focal length, because it is the distance between the lens and the light, but I could be wrong. Sorry I can't be anymore help.
From what I understood when I was doing this problem was that you are supposed to see an image on the actual sample (a laser). In a converging lens, the rays converging on one another is not the focal point, but where the image is formed.