The half-life of carbon-14 is 5730 years. explain how to figure out the age of a substance that has 25% carbon-14 remaining. a) multiply 25% times 5730 years since only 25% of the carbon remains. b) divide 5730 years by 2 since carbon has gone through two half-lives. c) multiply 5730 years by 2 since two half-lives have gone by for carbon. d) multiply 4 times 5730 years since carbon has gone through 4 half-lives.