What's the number of the episode seen by 2.24 million people in the US?
CREATE TABLE "table1_21696800_1" (
  "no" real,
  "num" real,
  "title" text,
  "directed_by" text,
  "written_by" text,
  "u_s_air_date" text,
  "production_code" text,
  "u_s_viewers_million" text
);
SELECT "num" FROM "table1_21696800_1" WHERE "u_s_viewers_million"='2.24';