who are the directors of the episode that had 20.5 millions of north american viewers? 
CREATE TABLE "table1_27714573_1" (
  "no_in_series" real,
  "no_in_season" real,
  "title" text,
  "directed_by" text,
  "written_by" text,
  "original_air_date" text,
  "production_code_s" real,
  "u_s_viewers_millions" text
);
SELECT "directed_by" FROM "table1_27714573_1" WHERE "u_s_viewers_millions"='20.5';