user: ### NOW ANSWER:
Question: What year was the Brabham BT46 BT48 BT49 Chassis common?
Columns: ['Year', 'Entrant', 'Chassis', 'Engine', 'Tyres', 'Points']
Types: ['real', 'text', 'text', 'text', 'text', 'real']
Sample row: [1961.0, 'Scuderia Serenissima', 'De Tomaso F1', 'Alfa Romeo Giulietta 1.5 L4', 'D', 0.0]
SQL:

assistant: SELECT AVG("Year") FROM "Table" WHERE "Chassis" = 'Brabham BT46 BT48 BT49';