The wording of the problem is a bit misleading. The scale is being accelerated upward with the person, imagine that the person and the scale are on a lift. The problem also assumes that you are near the Earth's surface, so you are experimenting a force equal to $F_z=-mg$ due to gravity. The normal force between the scale and the person is responsible for the person's upward acceleration. Due to Newton's Third Law, while the scale exerts a normal force over you (upward), you are exerting the same force over the scale (downward).
The scale measures the normal force you are acting over it, so we now know that the normal force is $N=2mg=400 pounds\cdot g$. Applying Newton's second law, we can deduce the acceleration:
Side note: your science teacher should really considerate teaching science using the SI.