Fermium-253 has a half life of 3 days. What percent of the sample is still Fermium-253 after 15 days?

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Explanation:

After every half-life, the quantity of the sample will become of the quantity at the beginning of the half-life. If the half-life for Fermium-253 is 3 days, it can be deduced that:

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Let . In general, after half-lives (for the fermium-253 in this question, that would be days,) the percentage of the sample that's still the initial substance would be:

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There are half lives in 15 days. (In other words, .) Therefore, the percentage of this sample that's still fermium-253 would be:

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