A person is standing on a weighing scale in an elevator in upward acceleration.
Let $N$ be normal reaction force exerted by the weighing scale to the person (upward).
It is known that the person will experience a normal reaction force ($N$) larger than his own weight (larger reading on the weighing scale). While $N$ is equal, but in opposite direction, to the force that the person exerts to the weighing scale due to action-reaction pair (the person and the weighing scale). That means other than the person's weight, there should be an extra force exerting to the weighing scale downward ($A$).
So $-N$ (upward)$ = mg + A$ (downward). I can't figure out where does $A$ come from?