What is the average week that there was a game with less than 18,517 fans attending and occurred on November 21, 1954?
CREATE TABLE "schedule" (
  "week" real,
  "date" text,
  "opponent" text,
  "result" text,
  "attendance" real
);
SELECT AVG("week") FROM "schedule" WHERE "attendance"<'18,517' AND "date"='november 21, 1954';