What is the average of the Series #s that were directed by Matthew Penn?
CREATE TABLE "season_3_1996_1997" (
  "series_num" real,
  "season_num" real,
  "title" text,
  "directed_by" text,
  "original_air_date" text,
  "production_code" text
);
SELECT AVG("series_num") FROM "season_3_1996_1997" WHERE "directed_by"='matthew penn';