12. Each edge of a regular tetrahedron is trisected, and through each division point, two planes are drawn, each parallel to the two faces of the tetrahedron that do not pass through this division point. How many parts do these planes divide the tetrahedron into?
(26th Moscow Competition Problem)
12.15 Part. 11 tetrahedra (1 at the top, 8 in the middle, 7 at the bottom) and 4 octahedra (1 in the middle, 3 at the bottom).