Table: congress_changes_7
Columns: state_class
vacator
reason_for_change
successor
date_of_successors_formal_installation

Question: How many reasons were given when A. Willis Robertson (D) resigned?
SQL: SELECT COUNT("reason_for_change") FROM congress_changes_7 WHERE "vacator" = 'A. Willis Robertson (D)'

Question: Who vacated his post when his successor was formally installed on May 11, 1966?
SQL: SELECT "vacator" FROM congress_changes_7 WHERE "date_of_successors_formal_installation" = 'May 11, 1966'

Question: What was the state (class) where the new successor was formally installed on May 11, 1966?
SQL: SELECT "state_class" FROM congress_changes_7 WHERE "date_of_successors_formal_installation" = 'May 11, 1966'