Four cards are placed on a table. Each card has a number on one side and a letter on the other. The cards on display show 3, 8, E and K. Which cards must be turned over to satisfy our requirement?
"If a card has a vowel on one side, then it has an even number on the other side."

This is an application of the Wason selection task testing the conditional rule: $P⟹Q$ (If Vowel, then Even number).
To verify this rule completely, we must check two cases:

1. **The card that satisfies $P$ (Vowel): We must turn it over to confirm it has an even number on the back. Out of the given cards, E is a vowel, so we must turn over E.
2. The card that violates $Q$ (Not an Even number / Odd number): We must turn it over to ensure that it does not have a vowel on the other side (which would break the rule). Out of the given cards, 3 is an odd number, so we must turn over 3**.

Note: Turning over '8' (Even) is unnecessary because the rule doesn't state that an even number must have a vowel on the other side. Turning over 'K' (Consonant) is also unnecessary because the rule only specifies what happens if a card is a vowel.

Therefore, we must turn over cards 3 and E.