If a rock contains an isotope with a half live of 600 million years, and 0.5 half lives have elapsed since the rock formed, how old is the rock in millions of years?

Answer 1

If 1.0 half life passed, it would be 600 million years. If 0.1 half life passed, it would be 60 million years. Since 0.5 half life has passed, it is 300 million years old.