I need help with this two quieries in sql.

here are the tables

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. Assume that small sections have less than 30 students, medium sections have at least 30 students but less than 80, and large sections have at least 80 students.

b. Find a list of courses (sections) where the percentage of students with D or F is greater than average.