Don't get scared and make something hard out of something easy.
Just think of " N " as one strong person.
The picture shows 6 strong people pushing the box to the right, and 5 strong people pushing it back to the left. (Like some kind of tug-o-war, except they're pushing, not pulling.)
You can see that the 5 pushers on one side exactly cancel 5 of the pushers on the other side, just as if all 10 of them are not even there at all, and the only thing left is the one 'net' person on the left.
The net force on the box is 1N, pushing to the right.
