Posted by Alejandro on Sunday, November 28, 2010 at 9:52pm.
Steven has 9 gold coins that are identical in appearance. However, one coin is counterfeit an weighs slightly less that the others. Using a balance scale, how can he find the counterfeit coin in just two weighings?

Math  MathMate, Monday, November 29, 2010 at 10:05am
Split the coins into three groups, A, B and C consisting of 3 coins in each group.
Weigh: AB
1. If A=B, the the counterfeit is in group C
Weigh any two coins in group C and figure out the rest.
2. If A≠B, then the counterfeit is in the lighter group.
Weigh any two coins in the lighter group and figure out the rest.