The function $\;\lfloor x\rfloor\;$ is "the integer part of $\;x\,$" or "floor function", which means: the greatest integer number _which is still less than or equal_ $\;x\;$ . So if $\;n=2k\;,\;\;k\in\Bbb Z\;$ is even , then
$$\left\lfloor\frac n2\right\rfloor=\lfloor k\rfloor=k=\frac{2k}2=\frac n2$$
Now you try to figure out the other one, taking into account that if $\;n\;$ is odd, then $\;n=2k+1\;$ .