To calculate the mass of an atom, you need to know the number of protons, neutrons, and electrons it has. The number of protons determines the atom's identity, while the combined number of protons and neutrons gives you its atomic mass.
In this case, you mentioned that the atom has 20 electrons and 20 neutrons. However, you didn't specify the number of protons. The number of protons determines the element, which determines the atomic mass.
If you provide the number of protons or specify the element, I can help you calculate the mass of the atom.