Four cards lie on a table. Each card has a number on one side and a letter on the other. Visible faces show: 'A', 'K', '4', and '7'. To test the rule 'If a card has a vowel on one side, it has an even number on the other,' which minimum set of cards must you flip?
'A' and '7'
The rule is an implication: Vowel -> Even. To test it, you must check if any vowel has an odd number (flip 'A'). You must also check if any odd number has a vowel hiding on the other side, which would disprove the rule (flip '7'). You do not need to flip 'K' (consonant, rule says nothing about consonants) or '4' (even number, rule allows vowels or consonants).
About this riddle
Logic riddles give you a few facts and ask what must be true. List what you know, cross out what cannot be, and see what is left.
Difficulty: Hard. More like this: Logic riddles with answers or try Einstein's Logic Grid for a different kind of puzzle.
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