Answer:
Let the distance between the origin and the 1st city =D =The distance between 2nd and 3rd city.
Let the distance between 1st and 2nd city = d,then:
The distance between origin and 1st city =1/3d + 100, and
The distance between 2nd and 3rd city =5/4d - 10. But we have:
1/3d + 100 = D..........(1), and
5/4d - 10 = D.............(2), but
1/3d + 100 = 5/4d - 10, solve for d
d = 120 miles - distance between 1st and 2nd city.
1/3 x 120 + 100 = 140 miles - distance between origin and 1st city.
5/4 x 120 - 10 = 140 miles - distance between 2nd and 3rd city.
140 + 120 + 140 = 400 miles - total distance travelled.