user: Question: What is the average capacity of stadiums in the City of london that belong to schools with an enrollment less than 30000?
Columns: ['Institution', 'Team', 'City', 'Province', 'First season', 'Head coach', 'Enrollment', 'Football stadium', 'Capacity']
Types: ['text', 'text', 'text', 'text', 'real', 'text', 'real', 'text', 'real']
Sample row: ['University of Windsor', 'Lancers', 'Windsor', 'ON', 1968.0, "Joe D'Amore", 13496.0, 'South Campus Stadium', 2000.0]
SQL:

assistant: SELECT AVG("Capacity") FROM "Table" WHERE "City" = 'london' AND "Enrollment" < 30000.0;