In the context of discussing the American education system, the more relevant figure is the number of native born functionally illiterate adults which, according to that source, is roughly 14%.
Almost 30 years ago, at my first job, a coworker got mad at someone and broke a broom. He was trying to write a letter to apologize and he couldn't. Couldn't meaning he was really trying but he didn't have the writing skills to write "I'm sorry that I broke a broom." The rest of the employees helped him.