This query orders tags by the number of closed (and non-deleted, obviously) questions as a percentage of total questions with that tag.
Q&A for people studying math at any level and professionals in related fields
FROM Tags AS t
INNER JOIN PostTags pt ON t.Id = pt.TagId
INNER JOIN Posts P ON p.Id = pt.PostId
where p.ClosedDate IS NOT NULL
group by t.TagName