# A rock sample contains 75 atoms of a parent isotope and 25 atoms of a daughter isotope. The half-life of the parent isotope is 100 years. How old is this rock?

##### 1 Answer

Oct 1, 2016

The rock is 41.5 years old.

#### Explanation:

For each half-life, you divide the total amount of the isotope by 2, so

You can rearrange this to

If there were originally 100 atoms, and 75 of the atoms remain, we have

The rock is 41.5 years old.