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.
