Among 100 coins exactly 4 are fake. All genuine coins weigh the same; all fake coins, too. A fake coin is lighter than a genuine coin.

How would we find at least one genuine coin using two weighings on a balance scale?

Source: 2010 Euler math Olympiad in Russia- authored by A.Shapovalov

(In reply to question for Kenny by xdog)

You are correct, but in this case, if you get two balanced situations in a row, you have used up all the "F"s, and it is a unique case, so you can merely pick a coin from the unused pile.  Thanks for contributing to the solution.

Posted by Kenny M on 2013-10-19 00:32:09