7 billion - challenge

Here's what I whipped up this morning, stemming from the ideas in the previous post.

I like the problem because we don't know what model the web site is using, so students can attack the problem in different ways and discuss the relative merits of each approach.  I also told them that I'd pay $20 if they end up predicting correctly (split between all the winners, of course).  I figure there's a good chance no one will.