SAT Subject Physics Practice Question 340: Answer and Explanation

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Question: 340

36. A sample of a radioactive substance has a half-life of 20 minutes. If the sample's activity is 200 counts/second, what is the number of counts/second after one hour passes?

A. 6.25 counts/sec
B. 12.5 counts/sec
C. 25 counts/sec
D. 50 counts/sec
E. 100 counts/sec

Correct Answer: C

Explanation:

The correct answer is (C). The sample has three half-lives occurring in one hour.. Next, we have . The original counts are multiplied by the amount ofthe sample remaining: .

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