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
"The trickier part is what to do when the original four coins DO balance the scale. The possibilities are:FF vs. FFRR. vs. RR"
You don't consider the possibility RF vs RF. Your next step gives RRFF on one side of the scale and potentially the same on the other, which is a dead end.
Posted by xdog
on 2013-10-18 21:27:49