x = lungimea traseului
x - ( 60 / 100 × x + 12 ) = 1 / 3 × x
x - ( 60 x / 100 + 12 ) = x / 3
60 x / 100 + 12 = x - x / 3
60 x / 100 + 12 = 3 x / 3 - x / 3
60 x / 100 + 12 = 2 x / 3
2 x / 3 - 60 x / 100 = 12
200 x / 300 - 180 x / 300 = 12
20 x / 300 = 12
20 x = 300 × 12
20 x = 3600 | : 20
x = 180 km