If there is matrix of $m$ rows and $n$ columns and another with $n$ rows and k columns, their product will be a matrix of $\mathrm{m}$ rows and $\mathrm{k}$ column. A is a non-null matrix with one row and 5 columns and $\mathrm{B}$ is a non-null matrix with 5 rows and one column. Therefore number of row in $\mathrm{A} \times \mathrm{B}$ is 1.