Question

Potassium-40 has a half-life of 1.3 billion years. If a rock was

determined to be around 3.9 billion years old what percentage
of the original potassium-40 would be left in that rock sample?

Answer 1

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.

Answer 2

Answer: 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.