A person weighing 800 newtons on earth travels to another planet with twice the mass and twice the radius of earth the person's weight on this other planet is most nearly?

The force of gravity is proportional to the mass, and inversely proportional to the SQUARE of the distance between them. If we double the mass at the same distance, we double the force; if we double the distance, the force is cut to one-FOURTH.

So twice the mass and twice the distance; the force will be (800*2)/2^2, or 400.