The starting amount of radioactive iodine-131 was 250 g. The half-life is 8 days. When Leslie measured the amount, she measured 15.625 g of radioactive iodine. How many days have elapsed between measurements?

The amount decreased from 250 g to 15.625 g, and 250 ÷ 15.625 = 16. Since 16 = 2⁴, four half-lives elapsed. Each half-life is 8 days, so 4 × 8 = 32 days; the correct choice is 32.

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