You then subtract this amount from the total amount of daughter atoms in the rock to get the number of decays that have occurred since the rock solified.
Here are the steps: a result of radioactive decay (call that isotope ``B'' for below).
Isotopes of a given element have the same chemical properties, so a radioactive rock will incorporate the NONradioactively derived proportions of the two isotopes in the Multiply the amount of the non-daughter isotope (isotope B) in the radioactive rock by the ratio of the previous step: (isotope B) × R = initial amount of daughter isotope A that was not the result of decay.
Subtract the initial amount of daughter isotope A from the rock sample to get the amount of daughter isotope A that IS due to radioactive decay.
By dating rocks of known ages which give highly inflated ages, geologists have shown this method can’t give reliable absolute ages.