The strange thing is, it's not always apparent whether someone is functionally illiterate or innumerate.

It's disturbingly routine, in the US at least, to have a degree with barely even a grade school understanding of math. And although total illiteracy is uncommon among high-school and college-educated people, you would be surprised how many can't write a normal paragraph or follow a written argument.

You can't necessarily see this with normal public or workplace interactions, but it is real.