So the rock is 1 half-life 1 half-life 1 half-life = 3 half-lives old (to get the age in years, simply multiply 3 by the half-life in years).
When plants absorb carbon-dioxide in the photosynthesis process, some of the carbon dioxide has the carbon-14 atom in the molecule.
Assuming that our atmosphere's composition and the cosmic ray flux has not changed significantly in the last few thousand years, you can find the age of the organic material by comparing its carbon-14/carbon-12 ratios to those of now-living plants.
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).
All atoms of an element have the same number of protons in their nucleus and behave the same way in reactions.