A rock sample is found to contain 25% of its original radioactive parent isotope remaining. If the half-life of the isotope is 500 million years, how old is the rock?

After one half-life (500 million years), 50% of the parent isotope remains. After a second half-life (another 500 million years), 25% remains. Therefore, two half-lives have elapsed, giving a total age of 500 × 2 = 1,000 million years. This is the principle of radiometric dating used to determine absolute rock ages.