Question: How many reasons were given when A. Willis Robertson (D) resigned?
SQL: SELECT COUNT("Reason for change") FROM "1-1847180-3" 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 "1-1847180-3" 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 "1-1847180-3" WHERE "Date of successors formal installation" = 'May 11, 1966';