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"`.