Counterfeit Money Problem

I came across this great problem and am thinking of using it to demonstrate the power of using a physical model in solving a math problem:

http://www.nctm.org/publications/blog/blog.aspx?id=42693&blogid=599514

and the answer:

http://www.nctm.org/publications/blog/blog.aspx?id=42820&blogid=599514