In the card game, the rule is that if a card has a vowel on one side, it must have an even number on the other side. Replace "the card has a vowel on one side" with "a person is drinking alcohol" and "it must have an even number on the other side" with "he or she must be over eighteen". Since a card with a vowel must have an even number on it, if a card has an odd number, one must check whether the other side has a vowel to know whether the card follows the rule. Similarly, if a person is under eighteen, one must check whether they are drinking alcohol to know whether they are following the rules.

If we refer to either "a card has a vowel" or "a person is drinking alcohol" as P and either "a card has an even number" or "a person is over eighteen" as Q, we can describe the four cards/people in both situations as:

1. P 2. ¬P 3. Q 4. ¬Q

The rule can be described as P implies Q. P implies Q is equivalent to "Q or ¬P". Therefore, in cases 2 and 3, P implies Q is known to be true without needing to know the value of the other proposition. In the remaining cases, you don't have enough information to know whether P implies Q. In case 1, P implies Q if and only if the value of Q is true when one checks it. In case 4, P implies Q is true if and only if the value of P is false when one checks it.