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?

a) 1.25 billion years ago

b) 2.5 billion years ago

c) 3.75 billion years ago

d) 5 billion years ago

Answer 1

the answer is b) 2.5 billion years. if its half life is 1.25 billion years and there is only 25% of the element remaining, you would take 1.25 billion and multiply it by two to get 2.5 billion.

Answer 2

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. 3.75 billion years ago was the rock formed. the answer is letter C