The half-life of a radioisotope describes the amount of time required for one-half of the isotope to decay into a daughter atom. After one half-life, 50% remains; after two, 25%, and so on.
To find how many half-lives have passed in 28,650 years, we can divide it by the time it takes for one half-life to pass. This reveals that five half-lives have passed.
HL ———— 1 2 3 4 5
% remain - 50 25 12.5 6.25 3.125
After 28,650 years or 5 half-lives, 3.125% of the original radioisotope remains in the fossil.
