A radioactive substance has a half-life of 5 million years. What is the age of a rock in which 25% of the original radioactive atoms remain? You must show your work or explain how you came up with your answer for full credit.
For a sample to decay to 25% of its original amount, 2 half lives must have passed. Since one half life is 5 million years, multiplying this by 2 gives us that 2 half lives is 10 million years.
