Among the establishments that failed in the inspection, what is the percentage of establishments with the highest risk level?
failed in inspection refers to results = 'Fail'; the highest risk level refers to max(risk_level); percentage = divide(count(license_no where risk_level = max(risk_level)), count(license_no)) * 100% where results = 'Fail'
SELECT CAST(COUNT(CASE WHEN T1.risk_level = 3 THEN T1.license_no END) AS REAL) * 100 / COUNT(T1.risk_level) FROM establishment AS T1 INNER JOIN inspection AS T2 ON T1.license_no = T2.license_no WHERE T2.results = 'Fail'