select count(d.author_id)  num
from (
select author_id,count(issue_type) cnt
from 
(
    select issue_type,author_id from answer_tb a,issue_tb b
	where a.issue_id=b.issue_id
	group by issue_type,author_id
	) c
where c.issue_type in ('Education','Career')
group by author_id
having cnt>=2
) d