Average If Neighboring Cell Contains Certain Value

I'm basically trying to create a gradebook for a class in which there are five categories of assignments, and each category is worth 20% of the final grade. I need to be able to calculate subtotals for each of the five categories as well as a final grade for the course. I have set up my workbook so that a code appears next to each grade that is entered. That code corresponds to the category as follows:

p = participation
o = organization
s = selfwork
g = OGT project
e = final exam

So I need a formula that will sum the values of any cells that have a "p" in the cell to the left, or any cells that have an "o" in the cell to the left, etc. I thought an array formula would work but apparently not if my range is made up of noncontiguous cells.

Here's how things look right now:Screen Shot 2014-06-26 at 10.21.36 PM.png

So, for example, I need a formula that will find that Kylie has earned 10 points on the OGT project (the green category).