Explanation:

- Let's denote the total distance as $D$.

- Half of the distance is $\frac{D}{2}$.

- Arpit covers $\frac{D}{2}$ at 30 km/h.

- Time taken to cover the first half: $\frac{D}{2}/30=\frac{D}{60}$.

- Arpit covers $\frac{D}{2}$ at 70 km/h.

- Time taken to cover the second half: $\frac{D}{2}/70=\frac{D}{140}$.

- Total time for the journey is 10 hours.

- Equation: $\frac{D}{60}+\frac{D}{140}=10$.

- Solve the equation for $D$:

$$\frac{7D}{420}+\frac{3D}{420}=10$$

$$\frac{10D}{420}=10$$

$$10D=4200$$

$$D=420$$

The correct answer is Option 4: 420 km.