user: Question: What is the average quantity for coaches manufactured in 1921/23, and had 40 seats?
Columns: ['Class to 1928', 'Class from 1928', 'Seats', 'Quantity', 'Year(s) of Manufacture', 'Remarks']
Types: ['text', 'text', 'text', 'real', 'text', 'text']
Sample row: ['Bi-21', 'Bi-21', '40', 10.0, '1921/23', 'Trial coaches']
SQL:

assistant: SELECT AVG("Quantity") FROM "Table" WHERE "Year(s) of Manufacture" = '1921/23' AND "Seats" = '40';