I'm not sure what the answer to the card one was, but I somehow knew "B" wasn't relevant, and that "A" was reasonable.
Because if I turned over "A," and found an odd number, the statement would be false.
I also knew that if I turned over "7," and found a vowel, the statement would be false.
However, my error was in assuming that if I turned over "4," and found a consonant, then the statement would be false. (Why did I think this? I guess I assumed that the statement could be reversed. "If a card has an even number on one side, then it must have a vowel on the other," and assumed that if the even number didn't have a vowel, then the statement was false. What kind of error is this?)
I already knew that "B" couldn't prove/disprove anything about the statement, because it was about vowels, and couldn't fall into the condition stated.
I was most positive of "A" and felt a bit shaky on the numbers, but not sure why.
So, I thought that you would have to turn over every card except "B" in order to know for certain whether the statement was true or false, but that my best bet for invalidating it would lie with "A."
I failed to eliminate "4," in other words.