AMO · 2027 · Grades 8-9 · Self-study SET 19 Question 7

Betsy collects sports posters. Exactly one of the following statements is false:
- Betsy has at least 11 sports posters.
- Betsy has at least 20 sports posters.
- Betsy has at least 24 sports posters.
- Betsy has at least 28 sports posters.

What is the greatest possible number of posters in Betsy's collection? Enter the integer only.