Below is an context that describes a sql query, paired with an question that provides further information. Write an answer that appropriately completes the request.  # noqa: E501
### Context:
CREATE TABLE table_18217750_1 (series__number VARCHAR, us_viewers__millions_ VARCHAR)
### Question:
What series number garnered 20.64 million viewers?
### Answer:
SELECT series__number FROM table_18217750_1 WHERE us_viewers__millions_ = "20.64"