# astronomy

The isotope potassium-40 (40K) decays into the isotope argon-40 (40Ar) with a half-life of 1.25 billion years. Suppose we find a rock in which seven-eighths of the potassium-40 has decayed to become argon-40, and only one-eighth of the original potassium-40 remains.

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1. 1/8 = (1/2)^3
so, 3 half-lives have passed.

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2. 2.6 billion years

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3. 69 hahahahaha

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4. 69.420 billion years lol ()

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