On a map 3 inches = 20 miles if the distance between two cities on the map is 15 inches how many miles apart are they

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".