user: Question: What series number garnered 20.64 million viewers?
Columns: ['Series #', 'Season #', 'Title', 'Directed by', 'Written by', 'Original air date', 'U.S. viewers (millions)']
Types: ['real', 'real', 'text', 'text', 'text', 'text', 'text']
Sample row: [48.0, 1.0, '" Listen to the Rain on the Roof "', 'Larry Shaw', 'Marc Cherry & Jeff Greenstein', 'September24,2006', '24.09']
SQL:

assistant: SELECT "Series #" FROM "Table" WHERE "U.S. viewers (millions)" = '20.64';