The isotope known as carbon-14 is radioactive and will decay into the stable form nitrogen-14. As long as an organism is alive, it ingests air, and the level of carbon-14 in the organism remains the same. When it dies, it no longer absorbs carbon-14 from the air, and the carbon-14 in the organism decays. The half-life of carbon-14 is 6000 years. (Note: There is a more exact value for the half-life in the textbook, but -- in working this problem -- you should use this value.)

a.) Some cave paintings in France are thought to be 18,000 years old. How many half-lives are in 18,000 years?

b.) Use the previous answer to determine what percentage of the original amount of carbon-14 remains after 18,000 years. (If necessary, round your answer to a tenth of a percent.)