A van is standing
Consider the persons and the van as a system. The center of mass of the system is at rest. The center of mass of this system should remain in rest as there is no external force to change its position. If the persons inside the van move in the backward direction, then the van has to move in the forward direction to keep the center of mass at the same position as it was before.
In this way, the van can be set in motion in the forward direction.
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