The half-life of carbon-14 is 5,730 years. how old is a sample that is 75% daughter isotope and 25% parent isotope?

Half-life it tells you about the amount of time needed that half of the quantity of an isotope to disintegrate.

For carbon-14, assuming that the daughter isotope is a stable one and does not disintegrate further, you have:

parent isotope           daughter isotope             years

100%                            0%                                     0

50%                             50%                                  5,730

25%                             75%                                  11,460

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