Wow.
Grade point average weights every class by how many units it's worth.
So say you got a C in a 3 unit lecture, and an A in the accompanying 1 unit lab.
A C is worth 2.0 on a 4.0 scale, and an A is 4.0.
The lecture is worth (2 grade points) * (3 units) = 6 points.
The lab is worth (4 grade points) * (1 unit) = 4 points.
You took a total of 4 units, so the average is (6 + 4 points) / (4 total credits) = 2.5.
Given how this forum's been today, now you'll probably bitch about how unfair this is...