What is the number in the series written by Steve Blackman and had a title of "Worlds Apart"?
CREATE TABLE "table1_24910737_1" (
  "no_in_series" real,
  "no_in_season" real,
  "title" text,
  "directed_by" text,
  "written_by" text,
  "original_air_date" text,
  "u_s_viewers_millions" text
);
SELECT "no_in_series" FROM "table1_24910737_1" WHERE "written_by"='Steve Blackman' AND "title"='\"Worlds Apart\"';