FACT: Each card has a number on one side and a letter on the other.
HYPOTHESIS: Every card that has a vowel on one side has an even number on its opposite side.
THE TEST: Which card or cards must you turn over in order to test the Hypothesis?
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The 15 possible answers are:
All four cards: A B C D
Three cards: A B C / A B D / A C D / B C D
Just two cards: A and B / A and C / A and D / B and C / B and D / C and D
Just one card: Card A only / Card B only / Card C only / Card D only