Martin Gardner's best mathematical puzzles

My dad had two of Gardner's books. I loved puzzling over them as a kid - enough that I bought my own copies for my kids to discover. I credit those books with much of my interest in logic.

Gardner's puzzles always had a beautiful simplicity to them. The solutions are sometimes fiendishly hard, but the problem is always accessible to the layman - no great grasp of mathematics is needed, just the ability to think logically and rigorously.

http://www.amazon.com/Mathematical-Logic-Puzzles-Dover-Recre...

http://www.amazon.com/Entertaining-Mathematical-Puzzles-Mart...

Has anyone got an answer for 7? I think I have the rest of them, (whether I'm correct or not obviously remains to be seen).