Several years ago, when I was teaching college-level Spanish, I had one class of students that were -- oh, let's say they were special.
I had to explain the concept of adjectives several times. And there was one lad who couldn't read the (fairly simple) paragraph in the book which explained them -- and yes, it was in English.
And (sigh) -- same class -- I have to say I was a bit surprised when they asked me how to figure out how they were doing in the class, because a full page of the syllabus was devoted to explaining this and providing them a chart they could use to fill in their grades as they got them. What caused them grief was the notion that the average of their four test scores would be used as a certain percentage of the final grade.
Why was this a problem?
There wasn't one person (of 20) in the class who was able to figure out the average of four two-digit numbers. One student said, "There's a button that says 'Average' on my calculator, but I don't know how it works."
So I taught them how to figure an average. And when I'm teaching math, somebody should be scared.