Physics help with camera lenses?

Question

I'm not totally sure about this question. I included it in the URL below with picture. I think an object can get as close as the focal length (uninclusive). So I said the distance had to be greater than 50mm (the focal length in the problem). Yet, I'm pretty sure I'm supposed to do that mathematically and don't know how. Could someone please show me? Thanks a lot.

http://www.flickr.com/photos/28859420@N07/2710741077/

Answer 1

Binoculars, rocks, and socks