Question

In front of you is 9 coins. They all look absolutely identical, but one of coins is fake. However you know that the fake coin is lighter than the rest and in front of you is a balance scale. What is the least number of weightings you can use to find the counterfeit coin ?
Brain Teaser

Answer 1

There is more than one way to solve this, here is what I would do.
I would label the 8 real coins x, and the fake light coin y .
The 9 coins :
x x x x x x x x y

Answer 2

hint : start by weighing 4 coins on each pan

Answer 3

There are two possibilites. The two pans balance or the two pans do not balance.

If the two pans balance, then you know you have " x x x x" is in pan 1 , and "x x x x" is in pan 2.
Therefore the coin you did not weigh is the light coin.

Now consider if the pans do not balance.