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After the first half-life, you will have one half of the starting amount. After a second half-life period, you'll be down to one quarter.

Of the part that radioactively decays, about 11% of it will decay to 40Ar, and the remainder to 40Ca.

Of your total sample of ordinary potassium, only 0.012% will be 40K. The half-life of 40K is about 1.3x109 years.

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11y ago
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14y ago

A 400 gram sample of any radioactive nuclide would decay to 100 grams after 2 half-lives.

AT = A0 2(-T/H)

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Q: How much of a 400-gram sample of K40 is left after 2 half-lives?
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