True, if you move something forward at 100 miles an hour but your on something moving backwards 100 miles an hour you up staying in the same location, aka zero velocity.
It's true and possible. Velocity over a period of time depends on the displacement during that time. Displacement is measured between the start point and end point. If, say, you're moving along a circular track, and you stop when you reach the point where you started from, your displacement is zero, so your average velocity for the trip around the circle would be zero.
