Question

A rock has 25 percent of its original amount of potassium-40 remaining in it; potassium-40 has a half-life of 1.25 billion years. How long ago was the rock formed?

Does anyone know how I can calculate this?

Answer 1

how many times can you cut something in half before you're left with 25% of what you started with?

Answer 2

2 times

Answer 3

right

Answer 4

so the "cutting in half" is the same thing as a half-life. Half-lives always leave behind half of what was present at the start of that half-life. Leaving 25% means that 2 half-lives have passed

Answer 5

since you know 2 half-lives have passed, and you know how long each half-life is, you can easily find out how long ago the rock was made (when it had 100% remaining, before any half-lives passed)

Answer 6

So what do I do with that?

Answer 7

that's your answer

Answer 8

But the possible answers are;
1.25 billion years ago,
2.50 billion years ago
5.00 billion years ago
6.25 billion years ago

Answer 9

since each half-life is 1.25 billion years, and 2 half-lives must have passed, how old is the rock?

Answer 10

2.5 then, ohh I see