user: Question: What is average quantity, when DRG number is 99 011?
Columns: ['Class', 'Railway number(s)', 'DRG number(s)', 'Quantity', 'Year(s) of manufacture', 'Axle arrangement ( UIC ) Bauart']
Types: ['text', 'text', 'text', 'real', 'text', 'text']
Sample row: ['L 1', 'XI–XXII, XXVIII', '99 081–99 092', 13.0, '1889–1907', 'C n2t']
SQL:

assistant: SELECT AVG("Quantity") FROM "Table" WHERE "DRG number(s)" = '99 011';