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briandmyers · 3462 days ago · link · · parent · post: A Three-Question Quiz To Test Your Rationality
In general, this is not true. You must turn over the '9' to verify the rule given these four cards.
Turning over the "2" doesn't help, because it may have a vowel or a consonant opposite, and you learn nothing either way, since both are allowed by the rule. [edit] In other words, you can never prove the rule, without examining all cards. The best you can hope for is to falsify the rule. Even if there were no odd numbers showing, you'd be in exactly the same position regarding the rule - it would be still be "true as far as I can tell".In order to unequivocally state that the rule is true, you have to then turn over a card with a number on it, because you've already proved that a vowel will be paired with an even number.