Based on the problem above, given can be translated into the following terms,
Distance from A to 1st city = 100+x
Distance from 1st - 2nd city = x
Distance from 2nd - 3rd city = x-10
Total Distance = 100+x+x+x-10
D=90+3x (equation 1)
Since A to 1st city equal 2nd to 3rd city, D -2(x-10) = x
D-2x+20 = x (equation 2)
Solving two equations with two unknowns simultaneously,
x = -35/3,
substituting to equation 1, the total distance traveled is 55 miles