“If there is a vowel written on one side of a card, then there is an even number written on the other side.”
How many of these four cards do you need to flip in order to check the validity of this sentence?
What would the answer be if you know that each card contains one letter and one number?
You need to flip all cards except for the second one. If each card contains one letter and one number, then you need to flip only A and 7.