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Problem 10 - Entrance Test

The half-life of a radioactive substance is 30 minutes. What fraction of the initial amount of the substance will remain after 1 hour?

Correct: B

After 30 minutes (1 half-life), 1/2 remains. After another 30 minutes (total 1 hour, or 2 half-lives), (1/2) * (1/2) = 1/4 remains.