The carbon in living matter contains a fixed proportion of the radioactive isotope carbon-14. The carbon-14 in 1.00g of carbon from living matter has an activity of 0.250Bq. The half-life of carbon-14 is 5730. When a plant dies the proportion of carbon-14 decreases due to radioactive decay. A 1.00g sample of carbon from an ancient boat has an activity of 0.160Bq. Determine the age of the board. Here's how I solved it... Original Activity = 0.25Bq Activity of Sample = 0.16Bq Then I just calculated what's that as a ratio of the original activity... 0.16/0.25 = 0.64 Then multiplied the half time by this number: 5730 * 0.64 = 3666 years ~ 3700 years Which is the correct answer. However this seems like a bit of a fluke. Especially since I've got a feeling I should be using this formula: x = x(original) ^-(lamda)*(time) Can anyone put my mind at ease, was my answer a fluke or is that a valid method to calculating the answer?