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The half-life of radium is 1600 years. The fraction of a sample of radium that would remain after 6400 years is

1. 1/4
2. 1/2
3. 1/8
4. 1/16

Correct answer: 1/16

Solution

The half-life of radium is 1600 years. After 6400 years, which is 4 half-lives (6400 ÷ 1600 = 4), the fraction remaining is (1/2)^4 = 1/16.

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