Final answer:
The journey from Gramton to Halfield takes 3 hours and 30 minutes. The car's average speed is calculated as 90 km/h for a distance of 315 km covered in the time taken.
Step-by-step explanation:
To find out how long the journey takes, we calculate the time interval between departure and arrival. Leaving Gramton at 16:39 (4:39 PM) and arriving at Halfield at 20:09 (8:09 PM), we see that the journey takes 3 hours and 30 minutes.
The car's average speed can be found by dividing the distance by the time. Since the distance is 315 km and the time is 3.5 hours (from the 3 hours and 30 minutes calculation), the average speed is:
Average speed = Distance / Time = 315 km / 3.5 h = 90 km/h.
Therefore, we don't need to convert this average speed since it is already in the unit of km/h, which is a standard unit for speed in the context of driving.