If $[x]$ denotes the greatest integer less than or equal to $x$, then the value of $\sum_{r=1}^{100} \left[ \frac{r}{5} \right]$ is:
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Recognize the periodic nature of division steps: $[r/d]$ repeats $d$ times for each integer value except at the boundaries. Here, there are $5$ copies of each integer from $1$ to $19$, with a single $20$ at the end.