Answer
d. 40,000
Work Step by Step
The half-life of a radioisotope is the amount of time it takes for half of the atoms in a sample to decay. If the half-life of the radioisotope is 20,000 years, then it would take 20,000 years for half of those atoms to decay. For another half of the original sample that had not decayed to decay, it would take another 20,000 years. Half of the original sample plus half of the half that had not decayed (which is one-quarter of the original sample) equals three-quarters of the original sample. Therefore, three-quarters of the sample will have decayed by 40,000 years.
