How many of the first 60,000 customers from the place with the highest average income per month have sent a true response to the incentive mailing sent by the marketing department?
place with the highest average income per month refers to GEOID where MAX(INCOME_K); RESPONSE = 'true';
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 T2.RESPONSE = 'true' ORDER BY T3.INCOME_K DESC LIMIT 1