Question: Among the customers from a place with more than 20,000 and less than 30,000 inhabitants, how many of them are Machine-op-inspcts?
SQL: SELECT COUNT(T1.GEOID) FROM Customers AS T1 INNER JOIN Demog AS T2 ON T1.GEOID = T2.GEOID WHERE T1.OCCUPATION = 'Machine-op-inspct' AND T2.INHABITANTS_K > 20 AND T2.INHABITANTS_K < 30