Hey ! Math geniuses! I need help.

[Vector]

Riddle: You have twelve pennies. One penny is a phony and has a different mass. You can't tell which penny is the fake.
You have a scale and are allowed three measurements, only.
As the riddle was related to me, I do not know if it is a balance scale or a scale that measures mass.
Can you determine which penny is the fake? If so, how?

 

[sasmith350]

make 3 groups of 4 pennies, balance 1 group against another, if they balance,take the third group and divide it into 2 groups of 2. if they don't balance take the group that doesn't and divide it into 2 groups of 2... ( of the group that doesn't balance)take the pairs and balance against each other, the lighter group has the fake penny, just balance them against each other to find out which one it is...

 

[dsa87gn]

answer

http://home.att.net/~numericana/answer/weighing.htm#weighing