Below is the list of tables from the database along with their columns and datatype. Need to generate an accurate SQL query for the question.
Only use the columns present in the respective table. Only use the columns which are required to generate the SQL query. Only use table joins if required and not otherwise.

### Tables ###
TABLE: CREATE TABLE table_15472061_1 (no_in_series VARCHAR, us_viewers__millions_ VARCHAR)

### Question ###
Name the number in the series for when the viewers is 7.78

### Accurate SQL ###

SELECT no_in_series FROM table_15472061_1 WHERE us_viewers__millions_ = "7.78"