A ¹⁴C sample has 25% of its original activity. How many half-lives have passed?
Q1. A ¹⁴C sample has 25% of its original activity. How many half-lives have passed?
Answer: 2
Explanation: Activity halves per half-life. 25% = (1/2)² = 2 half-lives. Option B miscalculates as three.