Answer
$d=\frac{1}{4\pi nr^2}$, means that $d$ varies inversely with $n$ and also with the square of $r$, and the constant of this variation is $\frac{1}{4\pi}$
Work Step by Step
The general equation for inverse variation is $y=\frac{k}{x}$, where k is the constant of this variation.
Here, $d=\frac{1}{4\pi nr^2}=\frac{1/4\pi}{nr^2}$, means that $d$ varies inversely with $n$ and also with the square of $r$.
As both $n$ and $r^2$ are in the denominator there will be an inverse variation between $d$ and $n$; $d$ and $r^2$.
In the numerator $\frac{1}{4\pi}$ is a constant, this will be the constant of the variation.
Therefore, we can say that the distance traveled by a gas atom varies inversely with the square of its radius and with the number of atoms per unit volume.
