Since real numbers are closed with respect to division (i.e. the division between two real numbers is again a real number), the fraction [tex] -\frac{m}{n} [/tex] is a real number everytime both m and n are real numbers.
In other words, you can choose any real number m and any real number n, and their ratio will still be a real number.
Of course, you have to follow the only rule that you can't divide by zero, so the answer is more precisely
[tex] m \in \mathbb{R},\quad n \in \mathbb{R}\setminus\{0\} [/tex]
