The truck cover 80 km per 1 hour that is 80 km per 60 min.
For half the time, the truck will cover half the distance.
So, for 30 min (half of 60 min) the truck will travel 40 km (half of 80).
In general, we could have done the following:
The truck travels
80 km per 60 min, so:
80/60 km per 60/60 min
that is
4/3 km per 1 minute.
in 30 minutes, the truck will cover a distance of (4/3)*30=4*10=40 (min)
Answer: 40 min