A sample contains 10 point 5 g of the radioisotope Pb - 212 and 157 point 5 g of its daughter isotope, Bi - 212 How many half-lives have passed since the sample originally formed?

Assuming the daughter isotope formed from the original Pb-212, the sample started with 10.5 g + 157.5 g = 168 g of Pb-212. The fraction remaining is 10.5 ÷ 168 = 1/16. Since 1/16 = (1/2)^4, four half-lives have passed, so the answer is B, 4.

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