本次MySQL國開形考作業的答案如下：

1. 第一題：
SELECT COUNT(*) FROM score WHERE grade >= 60;
2. 第二題：
SELECT COUNT(*) FROM student WHERE gender = 'M' AND grade >= 80;
3. 第三題：
SELECT MAX(grade), MIN(grade) FROM score WHERE course_id = 'math';
4. 第四題：
SELECT COUNT(*) FROM (
SELECT student_id FROM score WHERE course_id = 'chinese'
INTERSECT
SELECT student_id FROM score WHERE course_id = 'math'
) AS common_student;
5. 第五題：
SELECT AVG(grade), gender FROM score 
JOIN student ON score.student_id = student.id 
GROUP BY gender;

以上答案僅供參考，如有不同意見請您斟酌參考。