A geographer found that the land area of Aruba is 75 square miles and the land area of Bermuda is 19 square miles. Based on these data, the geographer used the equation 75x + 19y =T to estimate the total number of residents, T, of the two islands. Which is the best interpretation of x in this context?

The correct answer is: The population density, in residents per square mile, of Aruba.

Given ,75X + 19Y = T ,where T is the total no.of residents .
We know that the only like terms can be added and results will be in the same units only 
so ,75 x and 19 y are also no.of residents when added to give the total no.of residents T.
As 75 square miles x = total no.of residents of Aruba
x will be ratio of total residents to total area (i.e population density of the area)
So x is interpreted as the population density, in residents per square mile of Aruba.