What was the original air date of series number 10?
CREATE TABLE "series" (
  "series_number" real,
  "number_of_episodes" real,
  "original_air_date" real,
  "dvd_region_2_release_date" text,
  "dvd_region_1_release_date" text
);
SELECT AVG("original_air_date") FROM "series" WHERE "series_number"=10;