Conjecture: All Cards with an Even Number on one side have a Vowel on the Other.
Thus, if the A had an odd number, like a 1, on the other side, it would be ok. If the A had an even number, like 2, on the other side, that would also be ok. The conjecture does NOT say that all vowels have even numbers. "All dogs have noses" does NOT imply that if it has a nose then it is a dog. We do not need to turn over the A, because the conjecture did not specify what is on the other side of an vowel.