a.
| 12:00 to 12:30 | Travelled 50 km in 30 minutes, so 100 km/h |
| 12:30 to 13:00 | Stopped |
| 13:00 to 14:00 | Travelled 50 km in 60 minutes, so 50 km/h |
| 14:00 to 14:30 | Stopped |
| 14:30 to 15:00 | Travelled 100 km back towards the starting point in 30 minutes, so -200 km/h |
b. The whole journey was 200 km (100 km out and 100 km back) in 3 hours, so the average velocity was 66.7 km/h.