What is the average Margin that has a Round of 13. (h)?
CREATE TABLE "home_and_away_rounds" (
  "round" text,
  "st_kilda_saints" text,
  "opposition" text,
  "their_score" text,
  "margin" real
);
SELECT AVG("margin") FROM "home_and_away_rounds" WHERE "round"='13. (h)';