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The half-life of 32 grams of a radioactive substance is 6.5 hours. How many grams of the substance remain after 19.5 hours?
  1. 16 grams
  2. 8 grams
  3. 4 grams
  4. 2 grams
  5. 32 grams
Explanation
Answer: 4 grams

The half-life of a substance is the amount of time required for the substance to decay to half of its original mass. In the context of the question, 19.5 hours represents three half-lives. Therefore, the substance will be reduced to (1/2)3 or 1/8 its original mass. 1/8 x 32 gives the correct answer of 4 grams.
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