In which county is the residential area with the highest average income per household located?
highest average income per household refers to Max(avg_income_per_household)
SELECT T2.county FROM zip_data AS T1 INNER JOIN country AS T2 ON T1.zip_code = T2.zip_code WHERE T2.county = 'ARECIBO' GROUP BY T2.county ORDER BY T1.avg_income_per_household DESC LIMIT 1