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

AT = A0 2(-T/H)

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

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C-14 decays consistently over time, by estimating how much was in the sample to begin with and comparing it to how much is there now we can calculate how old the sample is. We can get good estimates for how much C-14 was there before by comparing samples dated by other methods

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Thorium-234 has a half-life of 24.1 days. How much of a 100-g sample of thorium-234 will be unchanged after 48.2 days?

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2.66666666666667 grams

5/100 or 1/20

3,375 days

That would depend on the initial amount of the substance, as well as on its half-life.

1/16 of the original sample of any unstable element remains after 4 half lives.

Depends on what the sample is and how much of it you have.

It is the difference between sand running out of an hour glass and determining what time it is by how much sand is left. Radioactive decay happens at a steady rate. If you can determine how much of that radioactive isotope ought to have been in a sample at the start and you can measure how much is left, you can tell how much time has passed.

It is the difference between sand running out of an hour glass and determining what time it is by how much sand is left. Radioactive decay happens at a steady rate. If you can determine how much of that radioactive isotope ought to have been in a sample at the start and you can measure how much is left, you can tell how much time has passed.

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Hi, Each half-life means the mass of the sample has decreased by 1/2 its mass. Thus; After 1 half-life, 1/2 the sample has decayed. After 2 half-lives 3/4 of the sample has decayed. Hope this helps.

If a sample of radioactive material has a half-life of one week the original sample will have 50 percent of the original left at the end of the second week. The third week would be 25 percent of the sample. The fourth week would be 12.5 percent of the original sample.

That would depend on the mass of the sample, its temperature, and what the sample is made of.