CREATE TABLE table_21696800_1 (_number VARCHAR, us_viewers__million_ VARCHAR)

What's the number of the episode seen by 2.24 million people in the US?
SELECT _number FROM table_21696800_1 WHERE us_viewers__million_ = "2.24"
SELECT "_number" FROM "table_21696800_1" WHERE "2.24" = "us_viewers__million_"