user: ### NOW ANSWER:
Question: How many decile has a roll less than 20?
Columns: ['Name', 'Years', 'Gender', 'Area', 'Authority', 'Decile', 'Roll']
Types: ['text', 'text', 'text', 'text', 'text', 'real', 'real']
Sample row: ['Arrowtown School', '1–8', 'Coed', 'Arrowtown', 'State', 10.0, 498.0]
SQL:

assistant: SELECT AVG("Decile") FROM "Table" WHERE "Roll" < 20.0;