The discussion focuses on calculating the specific activity of natural rubidium chloride, which contains two isotopes: stable Rb-85 and radioactive Rb-87, with Rb-87 having an abundance of 27.83%. The specific activity is defined as the ratio of activity to mass, where activity is determined by the product of the decay constant and the number of atoms of the radionuclide. For rubidium chloride, the relevant atomic masses are provided, including A(r) for Rb and M(r) for RbCl. The formula for specific activity involves the total activity of the radionuclides in the numerator and the total mass of all nuclides in the denominator. Accurate calculations require careful consideration of the isotopic composition and their respective contributions to the overall activity.