4 playing cards are placed in front of you. Two are face-down, with one red back and one black back. Two are face-up, with one ace and one queen.
Which cards must you turn over to check that every ace has a black back?
You need to turn over the ace and the red-backed card:
- If the ace doesn’t have a black back, the rule is broken.
- If the red-backed card is an ace, the rule is also broken.
Note that both the face-down card with a black back and the queen can have anything on the other side without violating the rule.
Commentary
This is also known as the Wason selection task.