formular problems

hi everyone, im newbie. here my problem
This is score student's list.

average=(math+phy+che)/3
100 is max score
and 50 is pass
1. how many student that is fail up 2 subjects?
2. display name and *** of student that is fail up 2 subjects
3. display name and *** of student that is fail math and che
4. display name of female student that is top score

many thank for your help.

ps:sorry about my bad english.