The half-life of a radioactive isotope is 140 days. In how many days does the decay rate of a sample of this isotope decrease to one-fourth of its initial decay rate?
Correct answer: B. 280
- A. 35
- B. 280
- C. 105
- D. 210
Explanation
The decay rate decreases in the same way as the number of nuclei. To decrease to one. fourth (1/2)² of the initial rate, two half. lives must pass. Therefore, the time required is 2 × 140 days = 280 days.
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