Suppose $\displaystyle R(k) = \{(i,j): a_{i,j} \leq k \} $. Why does $\displaystyle |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 $\displaystyle k $ is a positive integer?