Question

We start with 400 atoms of Uranium-238. How many remain after 4.5 billion years?____

After 9 billion years? ____
After 13.5 billion years? ____

Answer 1

The half life of Uranium-238 is about 4.5 billion years.

Amount after:
4.5 billion years = 200 atoms
9 billion years = 100 atoms
13.5 billion years = 50 atoms