How many units of item no.5 were sold in store no.3 on average on the days when the max temperature exceeded 90?
item no. 5 refers to item_nbr = 5; store no.3 refers to store_nbr = 3;  when the maximum temperature exceed 90 refers to tmax > 90; average = Divide (Sum(units), Count(date))
SELECT CAST(SUM(T1.units) AS REAL) / COUNT(T1.`date`) FROM sales_in_weather AS T1 INNER JOIN relation AS T2 ON T1.store_nbr = T2.store_nbr INNER JOIN weather AS T3 ON T2.station_nbr = T3.station_nbr WHERE T1.store_nbr = 3 AND T1.item_nbr = 5 AND T3.tmax > 90