What fraction of carbon 14 remains in an animal frozen in a glacier 18000 years ago?

The formula we can use here is:

A = Ao e^ (-kt)

where A is the amount remaining, Ao is the initial amount, A/Ao is the fraction we need, k is the rate constant and t is number of years passed

First we need to find k using the half life formula:

t1/2 = ln 2 / k

The half life of C 14 is t1/2 = 5730 years

therefore k is:

k = ln 2 / 5730 years

k = 1.21 x 10^-4 years-1

Going back:

A/Ao = e^ (-kt)

A/Ao = e^ (-1.21 x 10^-4 years-1 * 18000 years)

A/Ao = 0.1133

So only about 11.33% remains.