1. A person does not have the same weight on each planet. The weight is the force with which the planet pulls the person towards the center of the planet. As the properties of different planets (like mass, radius) are different, the weight is different on different planets, for the same person.
The weight of a person is the product of mass and acceleration due to gravity on the planet. Weight is the gravitational force of attraction between the planet and the person.
As g = G M / R², where G = gravitational constant , M and R are the mass and radius of the planets.
As the ratio M / R² is different for different planets, the attraction of different planets is different for the same person or for an object of same mass.
2. For obtaining weight, one has to multiply the gravity factor with the weight on Earth.
My weight is 64 kg.
So my weight on Earth = 64 kg * 1 = 64 kg
Mercury = 64 * 0.4 = 25.6 kg
Venus = 64 * 0.9 = 57.6 kg
Moon = 64 * 0.17 = 10.88 kg
Jupiter = 64 * 2.5 = 160 kg
Saturn = 64 * 1.1 = 70.4 kg
Uranus = 64 * 0.8 = 51.2 kg
Neptune = 64 * 1.2 = 76.8 kg
Sun = 28 * 64 = 1, 792 kg