By how much did the illiteracy rate drop from 1871 to 1920?

Although most states had established public schools by the Civil War, many school-age children still received no formal schooling. The majority of students who went to school left within four years, and only a few went to high school. Between 1871 and 1920 there was a drop in literacy levels in the U. S. where it's estimated that by 1920 there was a 92% literacy level (21.6 Million) in the U. S compared to 80% (9.9 Million) in 1871. College enrollments rose to more than four times as colleges initiated significant changes in curricula and admission policies. The industrial development had changed the nation's educational needs. During this period thousands and thousands of adult immigrants attended night school to learn English and to qualify for American citizenship.