Explanation: Every element has a different amount of protons in its nucleus. It is the same as its atomic number on the periodic table. For example: Hydrogen has an atomic number of 1 because it has 1 proton in its nucleus, Helium has 2, Carbon has 6, Platnium has 78, and so on...
We can use to formula . Here, N(A) is the final amount. N0 is the initial amount. t is the time elapsed, and is the half life. Plugging in, we get the answer above.