Database: address
Question: Among the cities with alias Ponce, what is the percentage of cities with a country level FIPS code of less than 20?
SQL: SELECT CAST(COUNT(CASE WHEN T2.county_fips < 20 THEN T2.zip_code ELSE NULL END) AS REAL) * 100 / COUNT(T2.zip_code) FROM alias AS T1 INNER JOIN zip_data AS T2 ON T1.zip_code = T2.zip_code WHERE T1.alias = 'Ponce'