user: Question: How many millions of viewers watched episode 15?
Columns: ['No. in series', 'No. in season', 'Title', 'Directed by', 'Written by', 'Original air date', 'U.S. viewers (millions)']
Types: ['real', 'real', 'text', 'text', 'text', 'text', 'text']
Sample row: [98.0, 1.0, '"Rio" (Part 4)', 'Joe Chappelle', 'Sunil Nayar', 'September18,2006', '17.62']
SQL:

assistant: SELECT "U.S. viewers (millions)" FROM "Table" WHERE "No. in season" = 15.0;