a. This table outlines the stages of the journey.

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. Even though the whole journey was `200\ "km"` (`100\ "km"` out and `100\ "km"` back) in `3` hours, the displacement for the journey (the distance from the starting point) is `0\ "km"`.

So the average velocity is `0\ "km/h"`.

On the other hand, the average speed was `200/3 = 66.7\ "km/h"`.

In summary,

`"ave velocity" = "displacement"/"time"`

`"ave speed" = "distance"/"time"`