The number of parent atoms originally present is simply the number present now plus the number of daughter atoms formed by the decay, both of which are quantities that can be measured.
Since the planet Earth doesn't have a birth certificate to record its formation, scientists have spent hundreds of years struggling to determine the age of the planet.
Nevertheless, there is substantial evidence that the Earth and the other bodies of the Solar System are 4.5-4.6 billion years old, and that the Milky Way Galaxy and the Universe are older still.
The principal evidence for the antiquity of Earth and its cosmic surroundings is: Spontaneous breakdown or decay of atomic nuclei, termed radioactive decay, is the basis for all radiometric dating methods.
In the early 20th century, scientists refined the process of radiometric dating.
Earlier research had shown that isotopes of some radioactive elements decay into other elements at rates that can be easily predicted.