
Final Exams Help
Hey guys, finals are coming up pretty soon (Tuesday to be exact) and I just wanted some help in figuring out my grade. I am really bad at percentages and constantly get mixed up by them so I thought i would run it by cboard to see if any of you could help me out (even my math teacher couldnt exactly explain to us how these finals grades work). The problem is that my dad would KILL me if I dont get an A for the semester grade. As it is I got a B for second quarter in science and i am surprised he hasnt already killed me. So I basically need to make sure that i get an A for the semester.
If you dont know how finals work you take a certain percentage (say 45%) of your first quarter grade, the same percentage for your second quarter grade and 1002x (x being the percentage of the first and second quarter grades) of your finals grade to get your semester grade (so if you took 45% for first and second quarter grades, finals would be worth 10%). I am not 100% (pun not intended) sure that this is how it works as these are my first finals. What I wanted you guys to help me with, is to figure out what percentage i need to score on my science final to get a 90%.
For first quarter i got a 94% and for my second quarter I got 84%. Could you guys help me figure out what percentage I need to score on my final if I want a 90% and finals are worth:
A. 10% (First and second quarters = 45% each.)
B. 15% (First and second quarters = 42.5% each.)
C. 20% (First and second quarters = 40% each.)
If you can only help me out with one or two of them that would still be cool. Sorry if I am asking for too much, but thanks a LOAD guys for any help in advance. :)

If I understood your explanation:
A: 0.45*0.94 + 0.45*0.84 + 0.10*x >= 0.90 => You'll need 99%
B: 0.425*0.94 + 0.425*0.84 + 0.15*x >= 0.90 => You'll need ~96%
C: 0.4*0.94 + 0.4*0.84 + 0.20*x >= 0.90 => You'll need 94%
Please note that I haven't heard of this system before, much less calculated anything, so take these calculations for what they are.

Depends on if they round grades or not (i.e. does an 89.5 get rounded to 90).
If so, the for the 10% case, you'd need at least a 94.
If not, then you'd need a 99 (as Sangdrax pointed out).
For the others:
15% with rounding: 93.
20% with rounding: 92.

Wait, yes they do round, but how does my required score drop that much?

Do the calculations:
Where x is the fraction each quarter is worth:
(quarter 1)*x + (quarter 2)*x + (final exam)*(1  2x) = 89.5
Do the same calculations replacing 89.5 by 90, and you will see where these numbers came from.
Cheers