For me it depends on what the double digits are - if you ask me what 63 times 77 is, then let me write it out, because though I can do it in my head, I'll feel more confident about the answer and can get it faster with a pencil and something to write on (paper, desk, hand, rock, lab partner's forehead, et cetera...). For the record: 4851 :P

The one that my lab partner couldn't answer was 15x11 ... REALLY not difficult at the University level! I know that you rarely see anything above the number 3 in Calculus with all those symbols standing in for actual numbers, but... COME ON!