A 60 point 0 - g sample of fermium- 253 was placed in a sealed vessel 9 point 0 days ago. Only 7 point 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, and 7.5 is one-eighth of 60.0. Three halvings reduce the sample to one-eighth: 60.0 → 30.0 → 15.0 → 7.5 g. Those three half-lives took 9.0 days, so each half-life is 9.0 ÷ 3 = 3.0 days. The correct choice is 3.0 days.

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