Assume each card has a letter on one side, and a number on the other.
Rule: if a card has a D on one side, then it has a 7 on the other.
You can see four cards (one side of each):
|
D
|
F
|
7
|
5
|
|
Card #1
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Card #2
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Card #3
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Card #4
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In order to check whether the rule is true of these cards, which cards do you need to turn over?