a rock sample containing an isotope with a half-life of 28 million years has an initial mass of 384 grams. How old is the rock after two half-lives?
After one half-life, the mass of the rock sample will be halved from 384 grams to 192 grams.
After a second half-life, the mass of the rock sample will be halved again from 192 grams to 96 grams.
Since each half-life is 28 million years, after two half-lives the rock is 56 million years old.