See the statement implies thatYou have 5 cards

You must prove the rul by turning over as few cards as possible.

the rule is : If the letter on side of a card is a vowel , the number on the other side is odd.

"if the *letter* on side of a card is a vowel, the *number*

on the other side is odd.

So the contrapositive of the statement is

If the *number* on side of a card is not odd, then the *letter* on side of a card is not a vowel.

So your right a "card" with one side a vowel and one side a letter

would not follow the rule. But it also would not meet the definition of a card because the rule assumes to that a card has a letter on one side and a letter on the other side.