Explanation:

- Let's break down the journey:

- Half the journey is covered at 10 km/h.

- The other half is covered at 20 km/h.

- Let’s assume the total distance of the journey is 2D:

- Distance for the first half = D, Speed = 10 km/h

- Distance for the second half = D, Speed = 20 km/h

- Time taken for first half = Distance/Speed = D/10 hours

- Time taken for second half = Distance/Speed = D/20 hours

- Total time = D/10 + D/20 = 6 hours

- Solve the equation:

- Multiply everything by 20 to clear the fractions:

- 2D + D = 120

- 3D = 120

- D = 40

- So, total distance = 2D = 2 × 40 = 80 km

- 80 km is the total length of the journey.