Deciding whether this map is well defined.

Deciding whether this map is well defined.

If I have the map $f: \mathbb{Z}_{20} \rightarrow \mathbb{Z}_{15}$ where
$x + 20\mathbb{Z} \mapsto x + 15\mathbb{Z}$, I have to determine whether
this is well defined or not. After staring at it for only a couple
minutes, I am pretty sure it is not well defined. Now, I must find a
contradiction to back up my answer. Not sure how to do that though. Maybe
the wording of the problem is just confusing me.