SELECT good.Snum, AVG(Mark) AS MarkAverage
FROM
(
	SELECT set1.snum FROM
	(
		SELECT Snum FROM Enrollment,
		(
			SELECT Cnum, Term, Section FROM Enrollment GROUP BY Cnum, Term, Section HAVING COUNT(Snum) > 20
		) AS c WHERE Enrollment.Cnum = c.Cnum AND Enrollment.Term = c.Term AND Enrollment.Section = c.Section GROUP BY Snum HAVING COUNT(Enrollment.Cnum) > 5
	) AS set1,
	(
		Snum
		FROM Enrollment
		WHERE Mark > 70
		GROUP BY Snum HAVING COUNT(Enrollment.Cnum) > 10
	) AS set2,
	(
		SELECT Enrollment.Snum FROM Enrollment,
		(
			SELECT Cnum, Section, Term	FROM Enrollment GROUP BY Cnum, Section, Term HAVING COUNT(Snum) > 20
		) AS c where Enrollment.Term = c.Term AND Enrollment.Cnum = c.Cnum AND Enrollment.Section = c.Section  AND Enrollment.snum NOT IN
		(
			SELECT Enrollment.Snum FROM Enrollment,
			(
				SELECT Cnum, Section, Term	FROM Enrollment GROUP BY Cnum, Section, Term HAVING COUNT(snum) > 20
			) AS c where Mark < 85 AND Enrollment.Term = c.Term AND Enrollment.Cnum = c.Cnum AND Enrollment.Section = c.Section
		)
	) AS set3 WHERE set3.snum = set1.snum AND set2.snum = set1.snum
) AS good, Enrollment WHERE Enrollment.Snum = good.Snum GROUP BY good.Snum;