So you want to start off by drawing the BCC structure. Two squares. And we know that an eighth of an atom fits in each corner. Oh yeah, and one in the back. I'll shade these parts in to show you that it's been sliced. And with a BCC, it's a body-centered cubic. So in a giant atom fits inside this cube. So to find the atomic packing factor, again, we want to relate the R to the A. And the A's are all on the side. They're all equal. And what we can see that along the body diagonal, the atoms are touching. So what you can say is that there's one, two, three, four hours squared is equal to, we have one A squared, two A squared, three A squared. So A squared plus, this is kind of a 3D Pythagorean theorem. Now that you have this relationship, you can just solve for the A to R ratio. So 16 squared, three A squared. I'm going to flip that around. A is equal to root 16 over 3R, which is the same as 4 all over root 3R. And that is your atomic packing factor for BCC.