A meteorite contains 0.17 g of nickel-59, a radioisotope that decays to form cobalt-59. The meteorite also contains 5.27 g of cobalt-59. How many nickel-59 half-lives have passed since the meteorite formed?

The original amount of nickel-59 was the nickel still present plus the cobalt-59 produced by decay: 0.17 g + 5.27 g = 5.44 g. The fraction of the original nickel that remains is 0.17 ÷ 5.44 = 1/32, which equals (1/2)^5. Therefore, 5 half-lives have passed; the correct choice is 5.

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