## Wednesday, May 25, 2011

There are four cards lying on the table. In order, the cards show: E V 2 7. Each card has a digit on one side and a letter on the other side.

Which cards should you turn around to test the following statement: "when there is a vowel on one side of a card, then there is an even digit on the other side"?

1. E and 2 obviously.

2. E and 7

3. E,2,and 7

4. E and 2

5. E satisfies the condition; turn that card over to verify the conclusion.
V contradicts the condition; the other side of the card tells us nothing useful about the conclusion.
2 satisfies the conclusion; the other side of the card tells us nothing useful about the condition (the statement does not claim that only vowels have even digits on the other side).
7 contradicts the conclusion; turn that card over to contradict the condition.

6. I think it's E and 2.
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7. Only two of the cards make a difference, the E and the 7. Flip over the E to see if there is an even digit on the other side. If not, you have a contradiction.

Flip over the 7 to see if there is no vowel on the other side. If there is a vowel on the other side, the statement is not true.

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