#### Answer

The population density in 1940 was $17.3~people/km^2$

#### Work Step by Step

We know that $1~mi = 1.6~km$
Then: $1~mi^2 = 1.6~km\times 1.6~km = 2.56~km^2$
In 1940, the land area was $2,977,128~mi^2$. We can convert this area to units of $km^2$.
$2,977,128~mi^2 = \frac{2,977,128~mi^2}{1}\times \frac{2.56~km^2}{1~mi^2} = 7,621,447.68~km^2$
We can find the population density in 1940.
$\frac{131,669,275~people}{7,621,447.68~km^2} = 17.3~people/km^2$
The population density in 1940 was $17.3~people/km^2$