How many entries are shown for date of successors formal installation where successor is john w. walker (dr)?
CREATE TABLE "table1_225099_3" (
  "state_class" text,
  "vacator" text,
  "reason_for_change" text,
  "successor" text,
  "date_of_successors_formal_installation" text
);
SELECT COUNT("date_of_successors_formal_installation") FROM "table1_225099_3" WHERE "successor"='John W. Walker (DR)';