If ${\text{u}}_1\mathrm{and}{\mathrm{u}}_2$ are the units selected in two systems of measurement and ${\mathrm{n}}_1\mathrm{and}{\mathrm{n}}_2$ their numerical values, then 

Physical quantity (p) = Numerical value (n) $\times$Unit(u)

If physical quantity remains constant then

$\mathrm{n}\propto \frac{1}{\mathrm{u}}\therefore {\mathrm{n}}_1{\mathrm{u}}_1={\mathrm{n}}_2{\mathrm{u}}_2$