how many routes end in the United States?
select count(*) from routes where dst_apid in (select apid from airports where country = 'United States')
among them, how many fly from the United States?
select count(*) from routes where dst_apid in (select apid from airports where country = 'United States') and src_apid in (select apid from airports where country = 'United States')
what are their ids?
select rid from routes where dst_apid in (select apid from airports where country = 'United States') and src_apid in (select apid from airports where country = 'United States')