Before we write the sentence comparing the densities, let's first calculate them. The density is given by the ratio between the amount of cattle and the area of the country.
The ratio for United States is:
The density of cattle for United States is 19.989 cattle/square mile.
Doing the same calculation for the other country:
The density of cattle for Republica is 40.573 cattle/square mile.
To do a relative comparison, we can do it by finding the ratio between those densities.
A sentence that makes a relative comparison would be:
The density of cattle in United States is less than the density of cattle in Republica, representing only 49.27% of the density of cattle in Republica.