What is the average score of the establishments owned by the owner with the highest number of establishments?
average score refers avg(score); owner with the highest number of establishments refers to owner_name where MAX(COUNT(business_id));
SELECT AVG(T1.score) FROM inspections AS T1 INNER JOIN businesses AS T2 ON T1.business_id = T2.business_id GROUP BY T2.owner_name ORDER BY COUNT(T2.business_id) DESC LIMIT 1