SAT Subject Physics Practice Question 145: Answer and Explanation

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Question: 145

8. Tungsten-176 has a half-life of 2.5 hours. After how many hours will the disintegration rate of a tungsten-176 sample drop to its initial value?

A. 5
B. 8.3
C. 10
D. 12.5
E. 25

Correct Answer: B

Explanation:

After 3 half-lives, the activity will drop to (1/2)3 = 1/8 its initial value, and after 4 half-lives, it will drop to (1/2)4 = 1/16 its initial value. Since 1/10 is between 1/8 and 1/16, the time interval in this case is between 3 and 4 half-lives, that is, between 3(2.5 h) = 7.5 h and 4(2.5 h) = 10 h. Only B is in this range.

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