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A radioactive element has half life period 800 years. After 6400 years what amount will remain?

1. 1/8
2. 1/16
3. 1/256
4. 1/8

Correct answer: 1/256

Solution

The number of half-lives is calculated as 6400/800 = 8. After 8 half-lives, the remaining amount is (1/2)^8 = 1/256.

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