Question Detail
Question
The half-life of a radioactive substance is 10 minutes. A sample of the radioactive substance contains 2000 nuclei. How many radioactive nuclei were in the sample half an hour earlier?
Correct Answer
Option D is the correct answer.
Detailed Explanation
Half an hour is 30 minutes, which is 3 half-lives (since each half-life is 10 minutes). The number of nuclei doubles each half-life going backward in time. Going back in time: 2000 * 2 = 4000 (20 min ago), 4000 * 2 = 8000 (10 min before that), 8000 * 2 = 16000 (30 min ago). So there were 16000 nuclei half an hour earlier.
Hint
Go backward in time. The number of nuclei doubles for each half-life you go back.