I once worked for a guy who typed 7 + 4 into a calculator, after freezing for 1.5 secs trying to work it out in his head. It was in a "stressful" situation (not something extreme, we just were in a hurry), and I'm sure the guy could add those numbers in his head, generally... he owns his own business, after all. It took so much out of me to not move a face muscle.