You're discussing this prism, correct?

I'm a little unsure where 1/4 and 5/12 comes from in reference to DEC. Suppose the entire volume is 1. Then as pyramid, C-DEF shares the same height as the prism, the volume of pyramid C-DEF is 1/3. Hence, plane DEC divides the pyramid into two regions of volume 1/3 and 2/3.

As for how a 1/4 comes in to play, let's look back at C-DEF, which we could also think of as F-CED. That is, we can consider CED the base and F the apex, so the height goes from F to the plane CED. Thinking of it this way, note that pyramid F-CPQ shares this exact same height (think about triangles in 2D here). However, as P and Q are midpoints, the base CPQ has 1/4 the area of base CED, so the volume of pyramid F-CPQ is 1/3 * 1/4 = 1/12.

Hence, F-CPQ has volume 1/12 and the other portion F-DEPQ has volume 1/3-1/12 = 1/4. This should help get everything started. As a final hint, 1 - 1/12 - 1/4 - 1/4 = 5/12.