One of them told me one day "I don't understand graphs." I tried but it's impossible at that point to make up for an apparent total lack of at least high school level arithmetic/math.
Also state universities are where most of the public school teachers and administrators trained, so they probably deserve a little of the blame for the state of public schools.
If you were doing a PhD you were at the kind of university that has a PhD programme. Probably it was one of the 200 or so US universities out of a thousand that are selective, rejecting more students than they accept. I doubt there was a single student in your seminar who wasn’t in the top 30% by academic aptitude of their age group.
> According to the U.S. Department of Education, 54% of adults in the United States have prose literacy below the 6th-grade level.
https://en.m.wikipedia.org/wiki/Literacy_in_the_United_State...
Like this stuff isn't particularly complicated, I'm not a very good teacher but I did teach some undergrad seminars during my PhD and I think even my worst students were capable of understanding functions, graphs and trig.