After 3 half-lives, the initial activity will be reduced to ______ of the initial activity?

• A. 1/4
• B. 1/16
• C. 1/8
• D. 1/2

Answer: (C)

Half-life is the time needed for the activity to drop by half. After each half-life, the remaining activity is halved again. So after three half-lives, the remaining activity is (1/2)³ = 1/8 of the initial activity. Therefore the correct amount is one-eighth of the original activity. The other options would correspond to two, four, or one half-lives, respectively.