So the scale in part a would be 72 miles/6 inches = x miles/ 1 inch. Cross multiply and you get 6x= 72, so x = 12 miles. Now you now that 1 inch equals 12 miles.
Part B tells you that Foston is 4 inches from Liberty, and asks you to figure out how far it is from West Quail. The WHOLE distance is 6 inches ( Liberty to West Quail) so the distance from Foston to West Quail is 6-4= 2 inches.
Remember 1 inch equals 12 miles, so 2 inches will equal 24 miles
