Necessary condition for multiplication of a matrix XY ..number of columns of Matrix X must be equal to number of rows of matrix Y
let Matrix X=\[\begin{bmatrix}2\\4\\0\end{bmatrix}\] and Y= \[\begin{bmatrix}2&1&0&4\end{bmatrix}\]
Then XY=X=\[\begin{bmatrix}2\\4\\0\end{bmatrix}\] \[\begin{bmatrix}2&1&0&4\end{bmatrix}\]=\[\begin{bmatrix}4&2&0&8\\8&4&0&16\\0&0&0&0\end{bmatrix}\]
