A paleontologist estimates that when a particular rock formed, it contained 12 mg of the radioactive isotope potassium-40, which has a half-life of 1.25 billion years. The rock now contains 3 mg of the isotope. About how old is the rock?
A) 2.5 billion years
B) 5.0 billion years
C) 1.25 billion years
D) 0.3 billion years
E) 0.4 billion years

Question

Quizplus · Accepted Answer

The rock has gone through two half-lives to reduce from 12 mg to 3 mg (12 mg to 6 mg in the first half-life, and then 6 mg to 3 mg in the second half-life). Since each half-life is 1.25 billion years, two half-lives equal 2.5 billion years.

A Paleontologist Estimates That When a Particular Rock Formed, It