A rock sample contains 1/4 micrograms of the parent radioactive isotope K-40 and 3/4 micrograms of its daughter isotope Ar-40. If the half-life of this decay is 1.25 billion years, how old is this rock?

Question

Quizplus · Accepted Answer

The rock has 1/4 micrograms of K-40 and 3/4 micrograms of Ar-40, indicating that 3/4 of the original K-40 has decayed. This means 3 half-lives have passed (since 1/2^3 = 1/8 remains, and 1 - 1/8 = 7/8 has decayed). Therefore, the age of the rock is 3 x 1.25 billion years = 3.75 billion years.

A Rock Sample Contains 1/4 Micrograms of the Parent Radioactive