Radioactive Decay of Potassium-40

Potassium-40 is a radioactive isotope with a half-life of 1.25 billion years. This means that after 1.25 billion years, half of the original amount of potassium-40 will decay.

After two half-lives, which is a total of 2.5 billion years, only one fourth of the original amount will be left. Let's consider an example where we start with 12 micrograms of potassium-40.

After the first half-life (1.25 billion years), half of 12 micrograms will remain, which is 6 micrograms. After the second half-life (another 1.25 billion years), half of the remaining 6 micrograms will be left, resulting in 3 micrograms.

Therefore, after two half-lives of potassium-40 have passed, the amount of radioactive potassium-40 remaining will be 3 micrograms if the initial amount was 12 micrograms.