What was the average year for a coin that had a mintage smaller than 10,000?
CREATE TABLE "nhl_goalie_mask_coins" (
  "year" real,
  "theme" text,
  "artist" text,
  "mintage" real,
  "issue_price" real,
  "special_notes" text
);
SELECT AVG("year") FROM "nhl_goalie_mask_coins" WHERE "mintage"<'10,000';