Answer: 100 miles
========================================================
Work Shown:
(3 inches)/(20 miles) = (15 inches)/(x miles)
3/20 = 15/x
3*x = 20*15
3x = 300
x = 300/3
x = 100
Therefore, 15 inches on the map correspond to 100 miles in real life.
-------------------
We could approach the problem like this:
3 inches = 20 miles
5*(3 inches) = 5*(20 miles)
15 inches = 100 miles
In the second step, I multiplied both sides by 5 to turn the "3 inches" into "15 inches".