Please read the following table in Markdown format and then answer the question according to the table. Table cells in one row are seperated by '|', and different rows are seperated by '
'.

Table:
| Institution | Location | Founded | Affiliation | Enrollment | Nickname |
| --- | --- | --- | --- | --- | --- |
| University of Illinois | Urbana, IL | 1867.0 | Public | 42606.0 | Fighting Illini |
| Illinois State University | Normal, IL | 1857.0 | Public | 20859.0 | Redbirds |
| University of Iowa | Iowa City, IA | 1847.0 | Public | 20907.0 | Hawkeyes |
| University of Missouri | Columbia, MO | 1839.0 | Public | 30831.0 | Tigers |
| Missouri State University | Springfield, MO | 1905.0 | Public | 22785.0 | Ice Bears |
| Northern Illinois University | DeKalb, IL | 1895.0 | Public | 25313.0 | Huskies |
| Saint Louis University | St. Louis, MO | 1818.0 | Private/Catholic | 10633.0 | Billikens |
| Southern Illinois University Edwardsville | Edwardsville, IL | 1957.0 | Public | 13398.0 | Cougars |
Question:
What is the average enrollment of the Redbirds' school?
Answer:
20859.0