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Take the right one (Posted on 2013-10-18) Difficulty: 3 of 5
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

No Solution Yet Submitted by Ady TZIDON    
Rating: 3.0000 (1 votes)

Comments: ( Back to comment list | You must be logged in to post comments.)
question for Kenny | Comment 2 of 18 |
"The trickier part is what to do when the original four coins DO balance the scale.  The possibilities are:
FF vs. FF
RR. 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
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