The Math Genie appears while you are doing your math homework. He offers you a small pouch of nine coins.

"What's the catch?" you say.

He tells you, "Eight of these are rare and quite valuable. One of them is a counterfeit. It weighs more than the other coins. I've brought this old two-pan balance. If you can find the counterfeit in the minimum number of weighings possible, the coins are yours."

**Question:** What's the fewest number of weighings that will identify the counterfeit coin for certain? Describe the steps you would take and how you can be confident you have found the counterfeit.

**Extra:** Describe the fewest number of weighings it would take to find the counterfeit if you did not know whether it was lighter or heavier.

Use this Java applet from the *National Library of Virtual Manipulatives* to help you think about the: Coin Problem

Use the **submit** link below to get hints and also chances to revise.