Please calculate the average duration of trips started at South Van Ness at Market in 2015.
started at refers to start_station_name; start_station_name = 'South Van Ness at Market'; average duration = DIVIDE(SUM(duration), COUNT(duration));
SELECT AVG(duration) FROM trip WHERE start_date LIKE '%2015%' AND start_station_name = 'South Van Ness at Market'