CREATE TABLE table_18217750_1 (series__number VARCHAR2, us_viewers__millions_ VARCHAR2)

What series number garnered 20.64 million viewers?
SELECT series__number FROM table_18217750_1 WHERE us_viewers__millions_ = "20.64"
SELECT "series__number" FROM "table_18217750_1" WHERE "20.64" = "us_viewers__millions_"