Discussion
The radioactive iodine (131I) has a half life of eight days. The amount of 0.5 g sample left after 32 days would be :
Answer: C
The amount left is 31.25 mg.
32 days is four half-lives of 8 days, and each half-life halves the amount:
\[ \begin{aligned} \text{left} &= 0.5 \text{ g} \times \left(\tfrac{1}{2}\right)^{4} \\ &= 0.03125 \text{ g} = 31.25 \text{ mg} \end{aligned} \]
0.25 g remains after one half-life, 125 mg after two and 62.5 mg after three; those options stop counting too early.
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