Interesting Puzzle

[Quaalude]

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I know, it was a joke. Hence the smiley.

But thanks anyway.

I hope everyone's having a good weekend.

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Oh, sorry. Sometimes it's hard to tell. [img]/images/graemlins/tongue.gif[/img]

[SheetWise]

But dropping them in water would provide enough resistance ...

[PairTheBoard]

Ok, I think I finally got this while falling asleep last night.

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First weight 4-4.
If they balance you're left with 4. Weigh 3 of those against 3 standards. If they balance, weigh the 4th against a standard. If they don't balance, weigh two of them against each other.

If the 4-4 don't balance you know either One 4-group is light or the other 4-group is heavy. Here's the tricky one. Put 3 lights and two heavies on the left scale. Weigh against 4 standards and 1 light on the right. If they balance you'll be left with 2 heavies which are easy. If the left is light you know it's the 3 lights on the left which you handle like above. If the left is heavy you know it's either the two heavies on the left or the one light on the right. Weigh the two heavies against each other and you're done.
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PairTheBoard

[PairTheBoard]

I like your innovation at the tricky part better than mine.

PairTheBoard

[PairTheBoard]

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Forgive me if this has been posted or if it is trivially easy. I found it interesting.

Your are given 12 marbles, identical in appearance. Exactly one marble has a mass either slightly more OR slightly less than the other 11 (the difference is so minimal no human could possibly detect it without the use of scientific equipment). You are given a balance and allowed to use it exactly 3 times. You must determine which marble has a slightly different mass and whether it has more or less mass than the other 11. How do you do it?

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I saw a more difficult version of this problem where you are not allowed to base your measurements on the result of previous measurements. Everything else is the same.

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I find it hard to believe it can be done with just 3 predefined weighings.

PairTheBoard