SELECT AVG(floors) FROM table_name_6 WHERE rank > 30 AND year < 1977 AND name = "100 van ness avenue"
What is the average amount of floors of the building that is ranked larger than 30, a year prior to 1977 and an address of 100 Van Ness Avenue?
CREATE TABLE table_name_6 (
    floors INTEGER,
    name VARCHAR,
    rank VARCHAR,
    year VARCHAR
)