If an astronaut had a mass 30 kg on the moon what would his mass be on earth?

Answer: 30kg

Explanation: It will still be 30 kilograms. A kilogram is a measure of mass, which is a measure of the quantity of matter in the object, not weight, which is how strongly gravity is acting on that object, so 30 kilograms is 30 kilograms regardless of the gravitational strength acting on it. You will weigh less on the Moon due to its lower gravity, but your mass is completely unaffected.