If a rock initially has 100 grams of parent isotope and 25 grams remain after some time, how many half-lives have passed?

• A. 1 half-life
• B. 3 half-lives
• C. 4 half-lives
• D. 2 half-lives

Answer: (D)

The amount remaining after a given time is halved with each half-life. Starting with 100 grams, one half-life reduces it to 50 grams, and a second half-life reduces it to 25 grams. Since you end up with 25 grams, two half-lives have passed.

For a quick check, one half-life would leave 50 grams, three half-lives would leave 12.5 grams, and four half-lives would leave 6.25 grams, so 25 grams fits exactly with two halvings.