In the 2005 season, how many races did the 15th place team complete?
CREATE TABLE "career_summary" (
  "season" text,
  "series" text,
  "team_name" text,
  "races" real,
  "poles" real,
  "wins" real,
  "points" text,
  "final_placing" text
);
SELECT AVG("races") FROM "career_summary" WHERE "final_placing"='15th' AND "season"='2005';