need help with this two quieries.

students(sid,name,age,gpa) // sid is the primary key

courses(cid,deptid, description) // cid,deptid is the primary key

professors(ssn,name,address,phone,deptid) // ssn is the primary key

enrollment(sid,cid,section,grade) cid makes reference to the courses table. // sid, cid, and section is the primary key

teaches(cid,section,ssn). cid, section makes reference to the enrollment table // cid, section is the primary key

a. List of professors that work for departments with more than 20 faculty members and that offer more large sections than small and medium sections combined.

b. Assume grades are A, B, C, D, F where D and F are failing grades. For each course (section) find the percentage of students that failed the course.