As potassium-40 decays, it becomes argon-40, as shown in the following graph.

potassium-40 has a half-life of approximately 1.25 billion years. approximately how many years will pass before a sample of potassium-40 contains one-fourth the original amount of parent isotope?
1.25 billion
2.5 billion
3.75 billion
5 billion

The answer to your question is  2.5 billion years

Explanation:

The initial amount of Potassium  There is                            X      

After 1,25 billion years                  There will be    

After 1.25 billion years                   There will be

That means that the after 1.25 billion years + 125 billion years there will be one-fourth of the initial amount.

Total time = 2.5 billion years