The ring ${\mathbb Z}/N{\mathbb Z}$ is usually defined in a rather cumbersome way, and it takes some time (infinite in most cases) before students realize that you can think of it as ${\bf Z}$ with just one added relation $N=0$ to do the computations (and the problem that $xy=0$ does not necessarily imply that $x=0$ or $y=0$), so that it is indeed a very simple object and not some horribly abstract invention.
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