Question

A rock formed with 2,000 atoms of a radioactive parent element, but only contains 500 radioactive parent atoms today. If the half-life for the radioactive element is one million years, how old is the rock

Answer 1

2 million years

Step-by-step explanation:

500/2000 = 1/4, meaning the sample decayed to 1/4 of its original amount. From this, we can conclude that 2 half lives have passed.

In the problem, we are given one half life is 1 million years. Multiplying this by 2, we get that 2 half lives is 2 million years.