Browsing around in my blog links, I found this:

Try it for yourself, and then:

**Can you explain how it works?**

You will have to use a little algebra. If you were at my pre-algebra workshop, this will be like the “Choose a number” problem.

**Can you explain why this puzzle will not work next year?**

What would you change to make it work in some year other than 2007?

**Can you make up a problem like this?**

Or can your students?

And if you get stumped, the answer is here.