CREATE TABLE table_15846 ("Order Year" TEXT, "Manufacturer" TEXT, "Model" TEXT, "Fleet Series  ( Quantity ) " TEXT, "Powertrain  ( Engine/Transmission ) " TEXT, "Fuel Propulsion" TEXT)

what is the fuel propulsion where the fleet series (quantity) is 310-329 (20)?
SELECT "Fuel Propulsion" FROM table_15846 WHERE "Fleet Series (Quantity)" = '310-329 (20)'
SELECT "Fuel Propulsion" FROM "table_15846" WHERE "Fleet Series (Quantity)" = '310-329 (20)'