I didn’t say we were universally good at it but in many countries we are (more or less) all exposed to it and taught it in school. There is a baseline even if we are far from mastery, and frankly a cynical hand wave of “well everyone’s bad at it” is not a real argument. In the US it is very clear that people are at least exposed to it and if we weren’t we would be operating very differently. I went to dinner last night, everyone did the tips in their head right there at the table. The average person can do some simple multiplication and division in their head. It’s not a mysterious script done on a box that they repeat the output of. Most people even if they’re bad at it understand the fundamental concept and, gun to their head, could probably do a series of very basic math problems with pencil and paper.

I imagine you and most of your peers can calculate a tip or at least approximate it without pulling out their phones yes?