A bank teller knows that one out of every eight coins is counterfeit and that the counterfeit coin will weigh less than a good coin. He has a balance (with two pans) to compare the weights of the coins. How can he identify the counterfeit coin with only two weighings? Thanks.
+129840 Put three coins on one side of the scale and three of the coins on the other side of the scale. Then either,
1) Both sides are out of balance and the side with the lighter counterfeit coin will "rise higer" than the other side.
Take the three coins offf the "higher" side and choose any two of them to weigh. If they balance, then the counterfeit coin remains. If they don't, then the counterfeit coin will "rise higher."
OR
2) Both sides balance. Then, the counterfeit coin is one of the two that remain and one more weighing will detect it.
