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.)