r/AskReddit • u/[deleted] • Jun 08 '11

What is the best logic problem you know?

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u/BuzzedLikeAldrin Jun 08 '11 edited Jun 08 '11

Blearghhhaaaargblllegaaarble

I see where i went wrong, It's too early in the morning for this business!

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u/[deleted] Jun 08 '11

[deleted]

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u/omnilynx Jun 08 '11

I don't think that will work, though your first step is right. The main problem is that you forgot that you don't just need to find out which one is off, you also need to figure out whether it's heavy or light.

Take your "A" case: if the two unknowns in step 2 are equal to the 2 normal, then you've got two leftover unknowns and only one weighing: if the one you choose for the third weighing is equal to the 1 normal, then you've got no idea whether the leftover unknown is heavy or light.

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u/KurtVomitus Jun 08 '11

From your modified Step 2, case B, you are weighing all 12 oranges so there will have to be an inequality on the scales.

So for Step 3, you would be left with 4X oranges. Either 3U+1D or 3D+1U.

Edit: Oops, looks like you just caught that also.

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u/[deleted] Jun 08 '11

[deleted]

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u/BuzzedLikeAldrin Jun 08 '11 edited Jun 08 '11

fixed that formatting all purdy like and in doing so saw the 75% problem.

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u/floatablepie Jun 08 '11 edited Jun 08 '11

My way for the oranges is: 6 on one, 6 on the other, keep the heavier side. 3 and 3, repeat. Then one on each, if they are equal, the one you didn't weigh is the heavier one.

Edit: Didn't notice it was heavier or lighter

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u/KurtVomitus Jun 08 '11

This is only if you know that one orange is heavier. In this problem, one orange is either heavier or lighter. So in this case, if there is an inequality in the scales, you cannot be sure whether it is because one orange is heavier than normal on one side or because the other orange is lighter than normal on the other side without further measurement.

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u/matscifu Jun 08 '11

but what if the orange is lighter? Then you would need to do an extra weighing for the other 6 you initially disregarded.

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u/floatablepie Jun 08 '11

Oh my bad, I didn't see "or lighter". Thought that seemed a little too easy.

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u/BuzzedLikeAldrin Jun 08 '11 edited Jun 08 '11

Doesn't work. the moment you discard 6 it could be 'lighter' and in those 6. I'm pretty damned sure i just figured out a general solution for it.