Mathematical Puzzles: What is () + () + () = 30 using 1,3,5,7,9,11,13,15? Why does the matrix [math] M = \begin{bmatrix}1 & 2  & 3\\4 & 5 & 6\\7 & 8 & 9\end{bmatrix} [/math] have a [math]0[/math] determinant?