Answer:
What percent of potassium-40 remains undecayed in a rock that is 3.9 billion years old? The 3.9 billion year old rock has undergone three half-lives (3.9 divided by 1.3 = 3). After 3 half-lives, 12.5% of the potassium-40 remains undecayed.
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Explanation:
If a rock was determined to be around 3.9 billion years old, Potassium 40 left in that rock would be 12.5%.
Half-life means that if the rock is 1.3 billion years, the Potassium 40 is reduced to half(50%).
2.6 billion years(twice of 1.3 billion years) rock would contain only 25%(half of 50%) of Potassium 40.
3.9 billion years(thrice of 1.3 billion years) would be left with only 12.5%(half of 25%).
Hence, the answer is 12.5%.
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