Answer:
2.5 grams
Explanation:
In radioactive decay, one half-life is the time it takes for half of the radioactive sample to decay. So if it takes two hours (for example) for a sample to go from 8 grams to 4 grams, then one half-life has passed (since half the sample has decayed), and the half-life is two hours.
Calculation:
2 half-lives = sample halved twice
10 grams / 2 = 5 grams (one half-life has passed)
5 grams / 2 = 2.5 grams (two half-lives have passed)
