Explanation :To determine the cardinality of the set of all matrices with rational entries, we first need to determine how many possible choices we have for each entry of the matrix. Since each entry can be any rational number, we can represent it as a fraction of the form p/q, where p and q are integers with For the first entry in the first row, we have a total of choices, where is the denominator of the fraction. Similarly, for the second entry in the first row, we have choices, and so on. Therefore, the total number of choices for the first row is: