When an object is thrown straight up from the ground and falls straight back down, the displacement of the object is zero because it returns to its initial position. Therefore, the average velocity over the round trip is also zero.
Average velocity is calculated by dividing the total displacement by the total time taken. In this case, since the object returns to its initial position, the total displacement is zero. The total time taken for the round trip would be the sum of the time taken for the object to reach the highest point and the time taken for it to fall back down.
Since the average velocity is zero and the displacement is zero, it means that the object covers the same distance upwards as it does downwards in the same amount of time. As a result, the average velocity is zero for the entire round trip.