user: Question: What was the average tonnage for the Siljan ship from Sweden?
Columns: ['Date', 'Name of ship', 'Nationality', 'Tonnage', 'Fate']
Types: ['text', 'text', 'text', 'real', 'text']
Sample row: ['17 October 1939', 'City of Mandalay', 'UK', 7028.0, 'Sunk']
SQL:

assistant: SELECT AVG("Tonnage") FROM "Table" WHERE "Nationality" = 'Sweden' AND "Name of ship" = 'Siljan';