user: Question: What source has a cardwell of 20%?
Columns: ['Source', 'Date', 'Cardwell', 'Jones', 'Knight', 'Lanier', 'Martin']
Types: ['text', 'text', 'text', 'text', 'text', 'text', 'text']
Sample row: ['Insider Advantage', 'July 8–9, 2008', '11%', '20%', '4%', '1%', '31%']
SQL:

assistant: SELECT "Source" FROM "Table" WHERE "Cardwell" = '20%';