Q&A for professional and enthusiast programmers
SELECT (CAST(Count(a.Id) AS float) / (SELECT Count(p.Id) FROM Posts p INNER JOIN Users u ON p.OwnerUserId = u.Id WHERE u.Reputation > 3000 AND PostTypeId = 2) * 100) AS AcceptedPercentage FROM Posts q INNER JOIN Posts a ON q.AcceptedAnswerId = a.Id INNER JOIN Users u ON a.OwnerUserId = u.Id WHERE u.Reputation > 3000 AND a.PostTypeId = 2