CREATE TABLE "table1_18217750_1" (
  "series_num" real,
  "season_num" real,
  "title" text,
  "directed_by" text,
  "written_by" text,
  "original_air_date" text,
  "u_s_viewers_millions" text
);

Question: What series number garnered 20.64 million viewers?
SQL: SELECT "series_num" FROM "table1_18217750_1" WHERE "u_s_viewers_millions"='20.64';