Among all the trips, which day had the most bikes borrowed? What was the average coldest temperature on that day?
which day had the most bikes borrowed refers to MAX(start_date); the average coldest temperature can be computed as DIVIDE(SUM(min_temperature_f), COUNT(min_temperature_f));
SELECT T2.date, AVG(T2.min_temperature_f) FROM trip AS T1 INNER JOIN weather AS T2 ON T2.zip_code = T1.zip_code GROUP BY T2.date ORDER BY COUNT(T1.start_date) DESC LIMIT 1