To determine the unknown element X, we first identify its atomic number and mass number. The atomic number is given as 20, which indicates that X has 20 protons. The mass number is given as 40, which is the total number of protons and neutrons in the nucleus. By subtracting the atomic number from the mass number (40 - 20), we find that X has 20 neutrons. Referring to the periodic table, the element with atomic number 20 is calcium (Ca). Therefore, the unknown element X is calcium.