/sci/ - Science & Math

This is in parts a homework problem, but the question I have is not really about you solving it, it is rather about understanding the notation. Having read the related chapter of my curriculum, the notation seems to be used exclusively in that one assignment, and no hint is given as to what it might mean. Now, assuming jsMath has \mathbb{} and I do not screw up the LaTeX:

Consider <span class="math">\mathbb{Z} \subset \mathbb{Q}[/spoiler] as abelian groups with + as composition. Let <span class="math">[q]=q+\mathbb{Z}\in\mathbb{Q}/\mathbb{Z}[/spoiler], where <span class="math">q\in\mathbb{Q}[/spoiler]. This is all well and good. The assignment, however, goes:

Show that <span class="math">\left[\frac{9}{4}\right][/spoiler] has order 4 in <span class="math">\mathbb{Q}/\mathbb{Z}[/spoiler].

What is the fraction supposed to mean? The notation is usually <span class="math">[a]=a+n\mathbb{Z}[/spoiler] of <span class="math">\mathbb{Z}/n\mathbb{Z}[/spoiler]. Should the number inside the square brackets not be an integer to make sense? Picture unrelated.