I first encountered humans who couldn't understand basic arithmetic at university, in the bit of first-year psychology where they try to bludgeon basic statistics into people's > heads. People who were clearly intelligent in other regards and not failures at life, who > nevertheless literally had trouble adding two numbers with a result in the thirties. I'm still boggling 25 years later, but I was there and saw it ...

See above, but I am basically one of those people. My own intelligence lies in other areas ;-)