What grade do I have to get on Final Exam to make a certain grade?

Wow! With this question floating around the internet so much around this time, I can't believe I can't figure this out! This is the basic question:

80% of my grades are in (for the 2nd semester) but I still have the 20% final left. How do I figure out what I have to make in order to get an "A" (which is 90-100), "B"(which is 80-90) or "C" (which is 70-80)?
For example, I made an 85 for the 80% part of the 2nd semester. So what is the formula for what I have to make on the 20% final in order to make each of those levels of grades?


And for an extra bonus, if I change the criteria to the 1st and 2nd semesters' grades are each worth 40%, and the final is worth 20%, THEN what do I have to get on the Final in order to make an "A", "B", or "C"?

Thanks.
Oh. and if you could hurry because it will impact on whether or not I do an all-nighter to study, or go out to my friend's bachelorette party!