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
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