Code:
A B C D E F
1 hw test proj part total
2 NAME .25 .5 .2 .05
3 A 90 80 80 90 83
4 B 80 80 80 80
5 C 70 60 63.3333
So here we have your Excel sheet. Top row is the categories, Left Column is the students. 2nd row is the weights assigned to each category. The formula you want in F3 is:
=
SUM((B3*B$2),(C3*C$2),(D3*D$2),(E3*E$2))/
SUMIFS(B$2:E$2,B3:E3,"<>")
There are two parts to this.
1 - The numerator is just the sum of the score for each category (B3) times the weight of that category (B2). Do this for each category (columns B through E). The $ just means that you are freezing row 2 so you can copy the formulas down to the following rows
2 - The denominator looks at each category cell (B3) for that student and if it isn't blank ("<>") it adds in the weight for that category (B2).
Numbers in red are what you should get from the formula