13.4 billion years is 3 times of the half-life, 4.47 billion years. So the Uranium-238 will go through three times of half decay. So the remain percentage will be 50%*50%*50%=12.5%.
The mass of an atom is given by the sum of the number of PROTONS and the number of NEUTRONS since ELETRON'S mass is negligible.
The mass is 24+28 = 52.
These are related but can be independent, because it is possible to, the theories are studied in mathematical logic, especially in model theory
Answer:
I think it might be a closed system
