If you have basic comfort in arithmetic, this is not a calculation that involves giving a damn. Being confused about why someone would give you an extra penny and having a discussion about it with a stranger burns 100x more calories than knowing it. If basic arithmetic involves taking a deep breath and closing your eyes for half a minute, or looking around the room for a calculator, that's a different cost/benefit analysis.

It's like the difference between a language you are fluent in and a language you are tentative in. If you're fluent, you have to make an effort not to listen to somebody's loud conversation, or not to pay attention to a billboard. They intrude into your consciousness. There's never a situation when I don't do simple arithmetic when exposed to it. I don't have to consciously figure out what 4 times 9 is. Subjectively, the number just pops into my head when I see the question.

edit: If you can't do this with explanations of identities or related rates, etc., it's hard or impossible to follow any quantitative or especially probabilistic argument. Even the simplest ones. I think this results in people for whom arithmetic is difficult faking it by trying to memorize the words used during quantitative arguments without having any real understanding. Just sort of memorizing a lot of slogans and repeating them during any argument that shares similar words. I think discomfort with arithmetic ruins people politically (as citizens), so I really do think calculators are a problem.