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Problem 2 - Olympiad

A nucleus undergoes α-decay with a half-life of 3.8 days. If 75% of a sample decayed, how many half-lives have passed?

Correct: B

The decay formula is N(t) = N₀(1/2)^(t/T), where 25% remains (since 75% decayed). Solving (1/2)^(n) = 1/4 gives n = 2 half-lives. Correct answer is 2.