An aircraft has an average speed of $600\ \text{km h}^{-1}$ on the outward journey and $400\ \text{km h}^{-1}$ on the return journey for the same distance. What is the average speed for the whole journey?
- A$111\ \text{m s}^{-1}$
- B$167\ \text{m s}^{-1}$
- C$480\ \text{km h}^{-1}$
- D$500\ \text{km h}^{-1}$