the book says to average the result, but do you average it by what you have, or what is possible to have

for instance, I have a full arm and torso, 10 points on the arm, 10 points on fron ttorso and 10 on the back torso

do i recieve +10 armor, +3 armor or +2.5 armor

You average across all the valid locations. IIRC, there are 6 (each arm, each leg, torso, head). Balancing front and rear torso armor prevents the need for writing a "but only X from behind" on your total. So, as I remember it, your two tens would be worth 3 and a third which usually ends up being 3.

Maybe someone with more memory of the cyberarmor would like to correct me IIDNRC.

It is averaged across five possible hit locations: arms, legs, head, front torso, and back torso.

So you would half the armor on the arm (because you only have one cyberarm with plating), then add it to the 10 points on front and 10 points on back, dividing the result by 5, which is 5 points.