I need help with this query 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. On average what percentage of students fail a course? (total number of students that failed a course / total number of enrolled students). A student failed the course if he/she gets a 'D' or an 'F'.