task SQL

SELECT a.dep_month,
       a.dep_day_of_week,
       AVG(a.flight_distance) AS average_distance
  FROM (
        SELECT dep_month,
              dep_day_of_week,
               dep_date,
               SUM(distance) AS flight_distance
          FROM flights
         GROUP BY 1,2,3
       ) a
 GROUP BY 1,2
 ORDER BY 1,2;