Final answer:
The remaining amount of potassium-40 after 2.5 billion years can be calculated by dividing the initial amount by the number of half-lives that have passed.
Step-by-step explanation:
The half-life of potassium-40 (K-40) is 1.25 billion years. To calculate how much remains after 2.5 billion years, we need to determine the number of half-lives that have passed in that time. We can do this by dividing the total time elapsed by the half-life of K-40:
Total half-lives = elapsed time / half-life = 2.5 billion years / 1.25 billion years = 2
Since 2 half-lives have passed, we can calculate the remaining amount of K-40 by dividing the initial amount by 2:
Remaining amount = initial amount / 2 = 130 g / 2 = 65 g