A radioactive sample of 640 g has a half-life of 3 days. How much remains after 9 days?
Answer and explanation
Correct answer: C. 80 g
- Step 1: Number of half-lives: 9 ÷ 3 = 3. Step 2. Remaining mass = 640 ÷ 2^3 = 80 g. Answer: 80 g. Why not the others: 320 g is what you get if you apply only one half-life. 213.333 g is what you get if you divide by the number of half-lives. 631 g is what you get if you subtract the days.
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