According to newton's second law of motion;
Force (weight of an object) = Mass × Acceleration
Weight of a 31kg object = 31kg × 10m/s² = 310N
Acceleration due to gravity on moon is less than that on earth so objects have little weight on moon than on earth.
The weight of a 31kg onject on moon is therefore
× 310N ≈ 51.7N
The mass of the object on earth is 31Kg.
In the formula; Force = Mass × Acceleration
Mass here is a constant on the acceleration changes so:
The mass of the object on moon is also 31Kg.