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