**Answer**

The truck travels the first 4 kilometres at 30 kilometres per hour.

Time taken = distance/speed = 4/30 hours = **8 minutes**

To travel 8 kilometres at 60 kilometres per hour

Time taken = distance/speed = 8/60 hours = **8 minutes**

**Therefore he can never average 60 kilometres per hour because he has no time left.**

(He would have to travel much faster for the first half of the journey.)