Below is an instruction that describes a task. Write a response that appropriately completes the request.

### Instruction:
What number episode in the series was watched by 15.89 million U.S. viewers?

### Inputs:
table_name: table, columns: ['No. in series', 'No. in season', 'Title', 'Directed by', 'Written by', 'Original airdate', 'U.S. viewers (millions)']

### Response:
SELECT No. in series FROM table WHERE U.S. viewers (millions) = 15.89