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