Can someone explain to me the general procedure used to identify and set up a proportion problem?

Question

Repost with a specific problem, so we can use it as an example.

I hope this helps a little. Thanks for asking.

The following ratios:
Map scale - distance between two points
75 miles (real): 2 inches (map)
=
n (actual distance between two points) : 3.4 inches(distance on the map between the 2 points)

I understand the equation and the solution, but need more formal way of explaining the general procedure used to identify and set up the proportion problem. Ex. why 75 is the numerator and 2 is the denominator same with n/3.4... 75/2=n/3.4