If the isotope decays for another ten years, what fraction remains?

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

Answer: (B)

Half-life decay means the amount left after each equal time interval is halved. If ten years is one half-life, the quantity remaining is multiplied by 1/2 each interval. If the isotope has 50% left at the start of this ten-year interval, another half-life leaves 25% remaining. So the fraction remaining is 25%.