The atomic number gives you the number of protons element x has. Since the mass of protons and neutrons are almost similar(around 1 amu), the mass number can be thought of as the sum of protons and neutrons. so if element x whose atomic number is 40 has a mass number of 82, then we know that 42 of those must be neutrons.
... directly over the Tropic of Capricorn, about 23.5 degrees south of the equator. The date is around December 22 or 23.
