Question

A rock has 12.5 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?


1.25 billion years ago

2.5 billion years ago

3.75 billion years ago

5 billion years ago

Answer 1

I think the rock would have 12.5% after three half lives - after one there would be 50%, after two there would be 25%

Answer 2

so it would be 2.5 billion years ago?

Answer 3

Sorry I didn't quite finish writing the reply - three half - lives would leave 12.5% so the answer is 3.75 billion years

Answer 4

oh ok..this is my last q's:
The half-life of a specific element was calculated to be 5200 years. What percentage of a sample of the pure element will remain after two half-lives?

it would be 25 percent?

Answer 5

that's right