美国文盲率21%，这是联合国和美国的官方公开数据_风闻 (guancha.cn)  

PS. I think the figure is close to the fact. During the eighties, I was working in LA. I read a report saying that about 25% of the graduates from high schools cannot comprehend the contents of the editorials in LA. Times.

FYI, I was shopping at a supermarket last week and saw an American couple was busy finding non-fat milk. Because no milk was labeled "non-fat" on the shelf, I pointed those "skim milk" to them. So, the couple might not know the meaning of "skim".