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An accident in a nuclear laboratory resulted in deposition of a certain amount of radioactive material of half-life 18 days inside the laboratory. Tests revealed that the radiation was 64 times more than the permissible level required for safe operation of the laboratory. What is the minimum number of days after which the laboratory can be considered safe for use?
- 64
- 90
- 108
- 120
Correct answer: 108
Solution
The radiation decreases by half every 18 days. Starting at 64 times the permissible level, it takes 6 half-lives (18 × 6 = 108 days) for the radiation to drop to the safe level.
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