Explanation:

- To solve the problem, let's denote the total distance as D km.

- First segment: One-third of the journey at 30 km/h. This means the distance is (1/3)D.

- Second segment: Remaining distance, which is (2/3)D, at 40 km/h.

- Time for first segment: (1/3)D / 30 hours.

- Time for second segment: (2/3)D / 40 hours.

- Total time equation: (1/3)D / 30 + (2/3)D / 40 = 12 hours.

- Solving this equation: D/90 + D/60 = 12.

- Common denominator is 180, thus: 2D/180 + 3D/180 = 12.

- 5D/180 = 12 leads to D = 432 km.

- So, the total distance is 432 km.