The example the OP gave is not even really a math problem. It's a complete lack of knowledge of how mixing different concentrations work.
This was precisely my thought -- knowing math is no substitute for understanding the concept that when two things blend, they average. As I said, my husband is better at math than I am -- which doesn't take a great deal of knowledge, really -- but he didn't get this concept either; he's never been a cook, or needed to blend things with differing levels of whatever active ingredient, so had never worked out the idea that averaging takes place between the active ingredients.
But again -- no matter how good she is at math, or with a cash register, someone unaware of this BASIC concept shouldn't be behind a vape counter!
Andria