If an isotope has a half-life of ten years and is allowed to decay for ten years, what fraction remains?

• A. Fifty percent remains
• B. Twenty-five percent remains
• C. Seventy-five percent remains
• D. Zero percent remains

Answer: (A)

In radioactive decay, the half-life is the time it takes for half of the material to decay. After one half-life, exactly half of what you started with remains. Since the elapsed time in this scenario equals one half-life, the remaining amount is half of the original, i.e., 50 percent. A quick cross-check: after two half-lives you’d have 25 percent left, and zero percent would require infinite time to reach, since decay approaches zero but never actually stops.