Yes, sorry!
select count ( * ) from student
How many students got an A?
select count ( t1.stu_fname ) from student as t1 join enroll as t2 on t1.stu_num = t2.stu_num where t2.enroll_grade = 'A'
Who got either an A or a C?
select count ( t1.stu_fname ) from student as t1 join enroll as t2 on t1.stu_num = t2.stu_num where t2.enroll_grade = 'C' or t2.enroll_grade = 'A'