Final answer:
If the mass of the Earth and everything in the solar system were doubled, a person's weight on Earth would also double. This is because weight is proportional to the mass of the Earth in the equation for gravitational force.
Step-by-step explanation:
The question is about how a person's weight would change if the mass of the Earth changed but the size remained the same. Weight is determined by the gravitational force between an object and a planet, which is given by Newton's law of universal gravitation. According to this law, the gravitational force, and hence the weight, is directly proportional to the mass of the planet. Therefore, if the mass of everything in the solar system were twice as much as it is now, and the sizes remained the same, a person's weight on Earth would double. Conversely, if the Earth had only one-third its present mass, a person's weight would be one-third what it is now. Likewise, if Earth had 10 times its present mass, a person's weight would increase by a factor of 10, assuming Earth's volume remained unchanged.