Average velocity =
(distance between start-point and end-point) / (time from start to end)
in the direction of
(direction from the start-point to the end-point) .
a). The average velocity for the first half is
(50m)/(20sec) = 2.5 m/s south.
b). The average velocity for the second half is
(50m)/(22sec) = 2.273 m/s north
c). Average velocity for the round trip is zero, because
the start-point and end-point are the same point.
However ...
Average SPEED = (total distance covered) / (time to cover the distance)
Average SPEED for the round trip is
(100 m) / (42 sec) = 2.381 m/s (rounded)