Answer: Ca
Explanation: the top number is the mass number which is protons+neutrons. The bottom number is the atomic mass which is just number of protons. In an isotope, the atomic mass will not change, the mass will making an isotope. So when you look on the periodic table you find the element with the same bottom number which is atomic mass.. since it doesn’t change in isotopes.. it should be the same. In this case, this isn’t an isotope because Ca has an atomic mass of 20 and a mass number of ~40