So it is easy to find number of edges given a half-square and half-cube by simply drawing, or just by intuition. We have two cases in both of these.
The square has 4 line of symmetry. If we define half-square by a line going through the edges, we can say half-square has 3 edges. If we define it by a line going through the sides, we can say half-square has 2 edges (or, well, 4). For a half-cube by the same logic we either have 6 or 4 (or 8).
The question is how do you generalize it to an n-cube? How do you even find how many different cases are you looking for? And even if you manage that, how can you determine how many edges does a half-n-cube has?