Question 20008: I tried solving this one yesterday but the answer I came up w/ can't be right, b/c it was too easy. I thought you would weigh all the coins together, then weigh them w/o the counterfeit coin, and the diffrence would be your answer, but here is the question.
In a group of 9 coins, one is counterfeit and weighs less than 8 genuine coins. Using a balance scale, determine the counterfeit coin in two weighings.
Thanks for your help.
Answer by wuwei96815(245)   (Show Source):
You can put this solution on YOUR website! Set three of the coins aside.
Weigh the remaining six coins (three on each side of the scale). This will tell you whether the counterfeit coin is one of the three that you set aside or with one of the sets of three that you are weighing.
If the coin is with the three that you set aside, weigh two of those coins to find the conterfeit coin.
If the coin is with one of the other sets of three (you should know which set from the first weighing), take two of those coins and weigh them. That will tell you which one of the three is the counterfeit coin.
So, it is possible to do it in just two weighings.
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