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The four cards

Pure logicLevel 2/5

This problem is a short, sharp lesson in what it means to check a rule. The winner is not whoever turns the most cards, but whoever rigorously distinguishes which ones could refute it.

There are four cards on the table. Each one has a letter on one side and a number on the other.

You see these faces: A D 4 7 You want to check if this rule is true: “If a card has a vowel on one side, then it has an even number on the other.” What cards do you need to turn over, and only those?

Solve this riddle

Hints

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  1. Don't look for examples that confirm the rule; Look for cases that can refute it.
  2. An “if P, then Q” implication only fails when P appears but not Q.
  3. Here eso means: a vocal with odd. By eso there are that revisar A and 7.

Solution

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Answer: You have to turn A and 7. Explanation: The rule says: > If a card has a vowel, then the other side has an even number. To verify it, you have to look for possible counterexamples. - A must be turned, because if there was an odd number behind it, the rule would be false.

  • D is not necessary: ​​the rule does not say anything about consonants.
  • 4 is also not necessary: ​​a card with an even number can have a vowel or a consonant behind it without violating the rule.
  • 7 yes, you have to turn it: if there was a vowel behind it, we would have a vowel with an odd number, and the rule would be refuted. That's why the only cards needed are A and 7.

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