What is average year for ayan mukerji?
CREATE TABLE "highest_grossing_bollywood_films_worldwi" (
  "rank" real,
  "movie" text,
  "year" real,
  "worldwide_gross" text,
  "director" text,
  "verdict" text
);
SELECT AVG("year") FROM "highest_grossing_bollywood_films_worldwi" WHERE "director"='ayan mukerji';