5. The six edges of tetrahedron $ABCD$ have lengths $7, 13, 18, 27, 36, 41$, and it is known that $AB=41$, then $CD=$
5. 13 Detailed Explanation: In a tetrahedron, except for $CD$, all other edges are adjacent to $AB$. If the edge of length 13 is adjacent to $AB$, let's assume $BC=13$. According to the triangle inequality, we know $AC \notin \{7,18,27\}$, thus $AC=36$, which implies $BD=7$, and $\{AD, CD\}=\{18,27\}$. In $\triangle ABD$, the sum of two sides is less than the third side, leading to a contradiction. Therefore, only $CD=13$. On the other hand, a tetrahedron $ABCD$ with $AB=41$ and $CD=13$ can be constructed, for example, by taking $BC=7$, $AC=36$, $BD=18$, and $AD=27$.