ACT Science Practice Question #1251 (Medium (31)) | Test Citadel
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ACT Science Difficulty: Medium (31)

Digital ACT Science Practice Question #1251

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A physics laboratory tested the radioactive decay of a sample of Isotope W. The initial activity was $1,600\text{ counts per minute (cpm)}$. After $36\text{ days}$, the measured activity had decreased to $100\text{ cpm}$. What is the half-life of Isotope W?
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Count the divisions by 2: $1600 / 100 = 16 = 2^4 \implies 4$ half-lives. Half-life $= 36 / 4 = 9$ days.

Masterclass Solution & Distractor Trap Analysis

Determine the number of half-lives that have elapsed: $1,600 \rightarrow 800$ (1) $800 \rightarrow 400$ (2) $400 \rightarrow 200$ (3) $200 \rightarrow 100$ (4) Four half-lives occurred over 36 days. Each half-life is $36 / 4 = 9\text{ days}...

Distractor Analysis: Trap choice eliminates careless test-takers who confuse roots with coordinates...

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