Slide 1 Let's see how you did answering this practice problem. This question asks you to calculate the age of a sample that has 25% of the original amount of C-14. Slide 2 The answer is D - 11,460 years old. So, why is this the correct answer? Slide 3 Organisms accumulate C-14 throughout their lives. At the time of death, an organism has a certain amount of C-14. After death, the C-14 begins to decay into N-14. So we can consider the time of death to be time 0. At t=0, the organism has 100% of its C-14 content. After each half-life, the organism loses 50% of its C-14 to radioactive decay. Slide 4 In other words, we can consider time 0 as 100% of the C-14 content. At time 1 (one half-life), the amount of C-14 is 50% of the original amount. Remember, one half-life loses half of the original C-14 to radioactive decay. At time 2 (two half-lives), the amount of C-14 is 25% of the original. The half-life of C-14 is 5730 years, so if a sample has 25% of its original C-14 it can be calculated to be 11,460 years old (this is calculated by multiplying 5730 years x 2). Actual radiometric dating calculations are more complicated but follow this same basic idea. If you have questions about how to solve problems like this one, please contact either me or a teaching assistant.