I'm going to use a specific notation for this one. It might seem a little confusing at first, but once you get used to it it saves a lot of time and trouble.

Define the symbol to be the force on object a that is coming from object b.

So Free Body Diagram time. In each FBD I'm defining upward to be positive. We don't need a horizontal axis.

For the box, we have a normal force (f represents the floor) pointing

up, a weight on the box (E is the Earth) acting straight down, and a normal force acting straight down.

So

(since the box is not accelerating.)

We need , so

So what is ?

Do a FBD on the person:

We have a normal force acting upward and a weight acting downward.

Thus

So

So the magnitude of the normal force on the person is equal to the magnitude of the weight of the person.

Now, to finish the problem we need to find , not . The notation is meant to suggest a relationship between the two: they are Newton's 3rd Law pairs. So by Newton's 3rd these have equal magnitudes. Thus

You can do the rest from here.

Edit: After going through all this it appears that the whole problem is that you gave your answer in terms of the mass, not the weight. Ah well, it was good for a demonstration.

-Dan