${\text{kg/m}}^3$ and${\text{g/cm}}^3$ are units of density and$\text{1000}{\text{kg/m}}^3=1{\text{g/cm}}^{\text{3}}$ By following the rule ${\text{N}}_{\text{1}}{\text{U}}_{\text{1}}={\text{N}}_{\text{2}}{\text{U}}_{\text{2}}$ ,we can say that${\text{kg/m}}^{\text{3}}$ is a smaller unit when compared to${\text{g/cm}}^{\text{3}}$

${\text{U}}_{\text{1}}={\text{kg/m}}^{\text{3}}{\text{U}}_{\text{2}}={\text{g/cm}}^{\text{3}}$

Density$=1000{\text{kg/m}}^{\text{3}}=1{\text{g/cm}}^{\text{3}}$  

$\text{P}={\text{N}}_{\text{1}}{\text{U}}_{\text{1}}={\text{N}}_{\text{2}}{\text{U}}_{\text{2}}$

Here${\text{N}}_{\text{1}}>{\text{N}}_{\text{2}}$ then${\text{U}}_{\text{1}}<{\text{U}}_{\text{2}}$  

So we get ${\text{kg/m}}^{\text{3}}$ smaller that${\text{g/cm}}^{\text{3}}$  

Therefore the given statement follows the rule$\text{N}\propto \frac{\text{1}}{\text{U}}$