user: Question: Name the sum of quantity for before 2011 model SLF-230
Columns: ['Year', 'Manufacturer', 'Model', 'Length (ft.)', 'Fuel or Propulsion', 'Fleet series', 'Quantity']
Types: ['real', 'text', 'text', 'text', 'text', 'text', 'real']
Sample row: [2002.0, 'Thomas Dennis', 'SLF-230', "30'", 'Diesel', '917–921', 5.0]
SQL:

assistant: SELECT SUM("Quantity") FROM "Table" WHERE "Year" < 2011.0 AND "Model" = 'SLF-230';