what is the last d5 1/2ns dose that patient 65962 was prescribed in 05/2103?
select prescriptions.dose_val_rx from prescriptions where prescriptions.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 65962 ) and prescriptions.drug = 'd5 1/2ns' and strftime('%y-%m',prescriptions.startdate) = '2103-05' order by prescriptions.startdate desc limit 1