user: Question: What is the average Year with Chassis equalling Cooper T60?
Columns: ['Year', 'Entrant', 'Chassis', 'Engine', 'Points']
Types: ['real', 'text', 'text', 'text', 'real']
Sample row: [1961.0, 'Louise Bryden-Brown', 'Lotus 18', 'Climax Straight-4', 0.0]
SQL:

assistant: SELECT AVG("Year") FROM "Table" WHERE "Chassis" = 'Cooper T60';