A car completes a journey at an average speed of 40km/h. At what speed must it travel on the return journey if the average speed for the complete journey is 60km/h?

The distance, s = 40t₁ where t₁ is the time to complete the outward journey.

The same distance = vt₂ where t₂ is the time to complete the return journey at speed v.

For the whole journey, 2s=60 (t₁+t₂). Therefore s=30 (t₁+t₂) = 40t₁=vt₂.

30t₁+30t₂=40t₁, 10t₁=30t₂, t₁=3t₂, so s=120t₂=vt₂ and v=120kph.