## Number of elements

Suppose $R(k) = \{(i,j): a_{i,j} \leq k \}$. Why does $|R(k)|= \left \lfloor \frac{k}{1} \right \rfloor + \left \lfloor \frac{k}{2} \right \rfloor+ \cdots + \left \lfloor \frac{k}{k} \right \rfloor$, where $k$ is a positive integer?