A sample originally contained 1.28 g of a radioisotope. It now contains 1.12 g of its daughter isotope. How many half-lives have passed since the sample originally formed?

Assuming the sample began as the radioisotope, the amount that remains is 1.28 g − 1.12 g = 0.16 g. That is 0.16 ÷ 1.28 = 1/8 of the original amount. Three half-lives reduce the parent isotope to 1/8, so the correct choice is 3.

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