A 60.0-g sample of fermium-253 was placed in a sealed vessel 9.0 days ago. Only 7.5 g of this isotope is now left. What is the half-life of fermium-253?

The sample decreased from 60.0 g to 7.5 g, which is one-eighth of its original amount: 60.0 ÷ 8 = 7.5. Reaching one-eighth takes three half-lives, since 60.0 → 30.0 → 15.0 → 7.5 g. The three half-lives occurred over 9.0 days, so each half-life is 9.0 ÷ 3 = 3.0 days, choice A.

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