What is the average money for a to par of 15, and is from the United States?
CREATE TABLE "final_leaderboard" (
  "place" text,
  "player" text,
  "country" text,
  "score" text,
  "to_par" real,
  "money" real
);
SELECT AVG("money") FROM "final_leaderboard" WHERE "to_par"=15 AND "country"='united states';