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# Four cards

2 replies to this topic

### #1 bonanova

bonanova

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Posted 03 September 2007 - 04:53 AM

The cards in a deck show either a circle or a square on their faces and have backs that are either red or green. Four cards are dealt. Two are face down, showing red and green; two are face up, showing a circle and a square, thus:

[card 1] RED
[card 2] GREEN
[card 3] circle
[card 4] square

Someone asks: Of those four cards, does every RED one have a square on the other side? How many cards must be turned over to gain enough information to answer the question? Which ones?
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The greatest challenge to any thinker is stating the problem in a way that will allow a solution.
- Bertrand Russell

### #2 jimmigee

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Posted 03 September 2007 - 05:10 PM

I'd say two: The red one, and the one with the circle. Obviously we don't care about the green one, we need to see if the red one has a square and we need to check that the circle is not red. As for the square, we don't really care about that either. If its red then we're fine, and if its green it's not relevant to the question.
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### #3 bonanova

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Posted 04 September 2007 - 03:22 AM

Yup.

The tricky part is to realize the square card
is a red herring, for exactly the reasons you gave.

And that comes from a careful reading of the question:
i.e. we're not asked if all the square cards are red.

Nice job.
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The greatest challenge to any thinker is stating the problem in a way that will allow a solution.
- Bertrand Russell

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