what was the total chlorpromazine dose that was prescribed since 15 months ago to patient 76275?
select sum(prescriptions.dose_val_rx) from prescriptions where prescriptions.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 76275 ) and prescriptions.drug = 'chlorpromazine' and datetime(prescriptions.startdate) >= datetime(current_time,'-15 month')