Assume each card has a letter on one side, and a number on the other.
Rule: if a card has a D on one side, then it has a 7 on the other.
You can see four cards (one side of each):
D
|
F
|
7
|
5
|
Card #1
|
Card #2
|
Card #3
|
Card #4
|
In order to check whether the rule is true of these cards, which cards do you need to turn over?