13. Two regular tetrahedra are inscribed in a cube such that four vertices of the cube serve as the vertices of one of them, and the four remaining vertices of the cube serve as the vertices of the other. What fraction of the volume of the cube is the volume of the common part of these tetrahedra?
13. The volume of the common part of the specified tetrahedra is $v=\frac{1}{6} a^{3}$, i.e., one sixth of the volume of the cube. The common part of the tetrahedra forms a regular octahedron.