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First consider \(r = 0\text{.}\) Here we are looking for all the numbers divisible by \(5\) since \(a = 5q+0\text{.}\) In other words, the multiples of 5. We get the infinite set

\begin{equation*} \{\ldots, -15, -10, -5, 0, 5, 10, 15, 20, \ldots\}\text{.} \end{equation*}

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