What is teh average rank of the city of mashhad, which had less than 667770 in 1976?
CREATE TABLE "cities_over_1_000_000_population" (
  "rank" real,
  "city" text,
  "province" text,
  "date_of_official_foundation_of_municipality" real,
  "2006" real,
  "1996" real,
  "1986" real,
  "1976" real,
  "1966" real,
  "1956" real
);
SELECT AVG("rank") FROM "cities_over_1_000_000_population" WHERE "city"='mashhad' AND "1976"<667770;