Fake Coin

By the way, did you know that Albert Einstein's brain was "normal" in weight? For the most part, it resembled an ordinary brain. There was, however, a slight difference. He had extra "cleanup" cells (called neuroglial cells). These cells move around the brain to get rid of dead or injured nerve cells. Perhaps his "well swept" brain supercharged his intelligence? You have nine gold coins. One of the coins is counterfeit and is filled with a lighter-than-gold substance. U sing a balance, what strategy can you use to uncover the counterfeit coin? To make things a little more difficult, you must identify the fake coin with only two uses of the balance.

Answer:

First, divide the coins into three groups of three. Then, balance anyone group against another group. If the counterfeit is contained in either of the groups, the coins will not balance. If, however, they balance, the counterfeit coin must be in the third pile. Now that we have identified the pile with the counterfeit coin, remove one coin from the pile and balance the other two. The lighter coin will not balance. If the two coins do balance, the counterfeit coin is the one not selected.