A sample of ²³⁸U (half-life 4.5 billion years) has 1/8th the original activity. How many half-lives have elapsed?
Q1. A sample of ²³⁸U (half-life 4.5 billion years) has 1/8th the original activity. How many half-lives have elapsed?
Answer: 3
Explanation: Activity halves every half-life. 1/8 = (1/2)^3, so 3 half-lives elapsed. Option B incorrectly adds one more half-life.