A sample contains 2.2 g of the radioisotope niobium-91 and 15.4 g of its daughter isotope, zirconium-91. If the half-life of niobium-91 is 680 years, how long will it take for the decay described in the previous question?

The original amount was 2.2 g + 15.4 g = 17.6 g. The 2.2 g of niobium-91 remaining is 1/8 of the original amount, which means three half-lives have passed (17.6 → 8.8 → 4.4 → 2.2 g). Three half-lives take 3 × 680 = 2,040 years, so the answer is C, 2,040 years.

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