Of the first 60,000 customers who sent a true response to the incentive mailing sent by the marketing department, how many of them are from a place with more than 30,000 inhabitants?
RESPONSE = 'true'; place with more than 30,000 inhabitants refers to GEOID where INHABITANTS_K > 30;
SELECT COUNT(T1.ID) FROM Customers AS T1 INNER JOIN Mailings1_2 AS T2 ON T1.ID = T2.REFID INNER JOIN Demog AS T3 ON T1.GEOID = T3.GEOID WHERE T3.INHABITANTS_K > 30 AND T2.RESPONSE = 'true'