How many restaurants failed the inspection in April 2010?
restaurant refers to facility_type = 'Restaurant'; failed the inspection refers to results = 'Fail'; in April 2010 refers to inspection_date like '2010-04%'
SELECT COUNT(DISTINCT T1.license_no) FROM establishment AS T1 INNER JOIN inspection AS T2 ON T1.license_no = T2.license_no WHERE strftime('%Y-%m', T2.inspection_date) = '2010-04' AND T1.facility_type = 'Restaurant' AND T2.results = 'Fail'