Computing Grades-getting average from a weighted test Ok well I have this basic problem in which there are grades (4 grades).  There's an upcoming final that is weighted to be some fraction toward the final grade (2/3). I have to find what the final grade has to be to get an average grade of 80. and then 90.  I completly forgot the procedure as to how to tackle this problem. Anybody have any hints/tricks for me to start me off?

The key to solving this problem is to realize that there are essentially two components that will go into the final grade : 
1) The average of the previous four tests
2) The grade on the final 
Thus we can set it up as follows : 
Let $G =$ Grade in the class, $a =$ average score from the previous 4 tests, and $f =$ final exam score. 
\begin{align*}
G = \frac{2f}{3} + \frac{a}{3}
\end{align*}
Using you're numbers you can solve for whatever possibilities you need.
EDIT: you can also use this approach for any different weightings by simply changing the fractional amounts that $a$ and $f$ are worth, for example if you want the final $f$, to be worth $3/4$ the final grade then it would reflect as: 
\begin{align*} 
G = \frac{3f}{4} + \frac{a}{4} 
\end{align*} 
