And that's the point. If I'm behind the counter at Kentucky Fried Doobies (assuming the War on Drugs ends someday), and I'm selling hash (just an example, relax), and I'm properly trained, I don't have to think to do that menial job, even when somebody gives me an odd amount of money.
Somebody standing there, puzzled, when they can simply enter the amount, indicates a greater problem than a math deficiency, IMO.