# How old is a mammoth's tusk if 25 percent of the original C-14 remains in the sample, if the half-life of C-14 is 5730 years?

##### 1 Answer

Apr 10, 2016

The tusk would be 11 460 years old.

#### Explanation:

You calculate the number of half-lives and multiply by the length of one half-life.

The number of half-lives is

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

You can rearrange this to

If the original amount was 100 %, and 25 % of the nuclide remains undecayed, we have