The Formation of a 1.25 Billion Years Old Rock

The Age of a Rock and Potassium-40 Dating

A rock is believed to have been formed 1.25 billion years ago, as calculated by using potassium-40 dating. Potassium-40 dating is a method used to determine the age of rocks by measuring the amount of potassium-40 and its decay product, argon-40, in the sample.

Potassium-40 is a radioactive isotope of potassium that undergoes decay over time. The half-life of potassium-40 is 1.25 billion years, which means that after 1.25 billion years, half of the original amount of potassium-40 will have decayed into argon-40.

Percentage of Original Potassium-40 in the Rock

Question: If the half-life of potassium-40 is 1.25 billion years, what percentage of the original amount of potassium-40 was found in this sample of rock?

Answer: The answer to the question is 50% of the original amount of potassium-40 will be left after one half-life or 1.25 billion years.
Explanation: To solve the question, we note that the half-life is the time for half of the quantity of the substance that undergoes radioactive decay to disintegrate. Therefore, after 1.25 billion years, 50% of the original amount of potassium-40 will be left in the sample.