What is the average median female age of all the residential areas in the Arecibo county?
"ARECIBO" is the county; average median female age = Divide (Sum(median_female_age), Count(country))
SELECT SUM(T1.female_median_age) / COUNT(T1.zip_code) FROM zip_data AS T1 INNER JOIN country AS T2 ON T1.zip_code = T2.zip_code WHERE T2.county = 'ARECIBO'