CREATE TABLE table_name_40 (capacity NUMBER, city VARCHAR2, enrollment VARCHAR2)

What is the average capacity of stadiums in the City of London that belong to schools with an enrollment less than 30,000?
SELECT AVG(capacity) FROM table_name_40 WHERE city = "london" AND enrollment < 30 OFFSET 000
SELECT AVG("capacity") FROM "table_name_40" WHERE "city" = "london" AND "enrollment" < 30 OFFSET 000 ROWS